Adios B&O MEXICO Tower

The reason I tend to miss a lot of closures and demolitions is because I don't tend to seek out news that will leave me bummed and depressed. In another example of bad news I missed the B&O's "M" aka MEXICO tower, near Cumberland, MD, was apparently demolished in September of 2023. The three story 1950's brick tower was similar to many across the B&O and, with its CPL bracket masts, had served as a backdrop to the local railfanning scene for decades.

CSX has been engaged in a long term project to convert the former hump classification yard to flat switching and a large obstacle to this was the physical hump and related structures.  MEXICO, built at the same time as the classification infrastructure, was located some 2 miles away at the eastern end of the yard leads and was in good structural condition.  Although its possible that CSX plans to alter the track configuration at MEXICO interlocking at some future date, I suspect that since they had already contracted for the asbestos remediation guys to come out for the yard hump tower, also getting rid of MEXICO could have come with little additional cost. 

Former Cumberland Yard hump infrastructure

Closed as an interlocking station in the mid-1990s, the relay based GRS plant was not re-signaled until the mid-2010's. Unless similar B&O towers at GREENE JCT (boarded up) and PATERSON CREEK (derelict), MEXICO saw use as an active railroad structure, either for maintenance crews or a rest facility for departing road freights.  The implementation of new service plans that generally sidelined the old system of classification yards may have played a role as Cumberland would see less originating eastbound trains and therefore less need for a standing structure to support the crews.

I personally only get to visit the location once on the ground, getting my photos from the road bridge like everyone else, although I had many opportunities for photos from the Capitol Limited with the last occurring in 2019.  Of those B&O towers that still survive, PATTERSON CREEK, only about 4 miles away from MEXICO as the crow flies, would be the most similar example.

NECR Resignaling Project is Complete

I can report that the re-signaling of the New Englande Centrale's main line that hosts Amtrak's Vermonter north of the Massachusetts border has been completed. While I had previously reported on the state of this project, I can confirm that all legacy signaling has now been replaced. While the diamond at Bellows Falls was perhaps the most visible of the final round of changes, the north end of the Putney siding was also re-signaled. 

As always there are a few interesting details I should point out. The previously reported signaling anomaly at Bellows Falls resulting from a old double to single track conversion, has been eliminated. Trains at Bellows Falls now get a Clear signal indication for a straight route at Wapole siding as opposed to the previous Approach Medium to Medium Clear. 

The holdout signal at SOUTH BELLOWS FALLS has been retained, evidently to allow interchange with the Green Mountain Railroad without needing to seize the main line all the way to Putney.

The entire NECR portion of the line has done away with the Boston and Maine / Guilford practice of ultra-long ABS blocks. Now signal blocks will be a more uniform 2-4 miles which limits the impact of track circuit failures and increases capacity.

Having been re-signaled just prior to the start of the most recent project, the South Putney interlocking will be the only non-standard signal location on the line with a poor man's bracket mast for southbound trains and the use of Safetran scallop shell modules as opposed to Progress Rail square modules.

Although an honorable mention goes to the old northbound searchlight signal on the Bellows Falls platform that was so popular as a photo backdrop. That has been moved to the adjacent park around the Bellows Falls Chamber of Commerce.

It will be interesting to see if this affects on time performance or the schedule at all. In theory replacing the TWC with CTC for the 10 or so miles south of Brattleboro could allow a speed increase.

No Approach! The Duality of Uncommon Signals

We all learn about traffic lights in elementary school.  Red means stop, green means go and yellow means slow or caution. These are the basics of automatic block signaling that also tend to get taught in entry level railroading books. Now its pretty logical that a signal might lack the ability to display a clear indication. A permanent stop condition or the end of automatic block territory are both situations where a Clear signal would not ever apply. However there are a few situations where signals in North America can display Stop or Clear, but not Approach on the full speed head. 

The first is at interlockings outside of automatic block territory like a diamond crossing or drawbridge. The signal provides movement authority through the interlocking and without track circuits outside of the interlocking limits there is no "prepare to stop" points an Approach type signal would apply to. Trains will have their own movement authority like a track warrant so a clear signal is basically the interlocking telling the train "you do you". One might expect signals in this case to display a Restricting indication, instead of a clear, and sometimes they do, but that is going to come with a 20mph speed restriction, or less, which can be a time hit on unsignaled lines with higher track speeds. 

The next situation, Manual Block territory, would be very familiar to those in other parts of the world. Although almost extinct in the US, there remains a bit of manual block still in service on the LIRR and the way to identify a manual block signal is the lack of an approach indication. (This also applies in general to historic PRR signaling charts.) Signals will display Stop or Clear Block with any Approach functionality handled by a separate distant signal towards the end of the manual block. The PRR even went one further sometimes substituting the \ "Caution" indication in place of / Approach. 

Sometimes its really not good to have locomotives stopped and waiting at a specific signal,  Maybe there's an issue with a grade crossing, or noise, or diesel exhaust. In this case it would not be desirable for trains to approach that signal prepared to stop, so a preceding signal will either hold trains short or allow them to approach a signal with some sort of proceed indication. The most notable of these is at the entrance to Washington Union Stations 1st St tunnel that the currently diesel hauled northbound trains cannot enter without having a signal displayed further on that allows them to leave said tunnel. Another nearby signal location at CSX's LENFANT interlocking might lack southbound approach signals for a similar reason due to an overbuild.

One somewhat PRR-specific situation is where interlockings are provided with exit signals and because of the short distance between the interlocking's entrance signal and exit signal, Slow Approach is used instead of Approach. This creates an upper head that can only display --- Stop or | Clear. The C&O also made frequent use of exit signals, but seemed to keep their upper head Approach indication, although Medium Clear to exit Approach allowed the C&O to minimize the use of R/Y/Y/ Medium Clear.

A fairly new situation where Approach has been "omitted" involves high speed turnouts. At certain points on the NEC, Amtrak has decided to combine a flashing green "Cab Speed" signal indication with R/*Y* Medium Approach. At the end of a main track that converges into another via a high speed turnout, the result will be an upper head with green and red lamps/positions and no yellow lamp/position.
 

The Enthusiasts Step Up

Unlike Europe, Heritage railroads in the United States never developed a vintage signaling component beyond what one might describe as display items. The big reason for this is that in service signaling and interlocking appliances have significant regulatory inspection, testing and documentation requirements that are beyond the reach of most tourist train operators. The result has been that most preserved towers are completely inert, with just a handful having interactive elements such as operable interlocking machines (SS43 BERK) or full on simulations (HARRIS). However, in recent years it seems that the private collector/enthusiast space has been stepping up to fill the gap as exemplified by the small YouTube channel Laser 711.

Signal and signaling equipment collectors have been around for some time, snapping up such items as model boards, CTC cabinets, full size signals and even interlocking machines. Until recently, railroad signaling departments would tend to keep their retired equipment, as similar vintage hardware was still broadly in service and the parts to maintain them were no longer being manufactured. However, the PTC related re-signaling push rapidly phased out so much of the relay era railroad signaling that the equipment is now being sold for scrap or offered free to anyone with the capacity to haul it away. 

In the context of interlocking towers, model boards and CTC cabinets have been widely collected for some time as switch to video display in a dispatch office had long ago destroyed their reuse value. Somewhat ironically, it was the older mechanical and electro-mechanical interlocking machines that retained the most value as a source of spares for their kin.  (For example a major reason LENOX tower near St. Louis was closed was to supply parts to the CNW LAKE ST machine in Chicago.) However as the number of these in service interlocking machines hit zero, anyone with a large enough truck could cart the survivors away for their own personal amusement. This is where Laser 711 comes in, having apparently set up not just a HARRIS style PLC backed simulation, but the full hardware stack including relays, point machines and signals.

 He has also restored an ex-Erie CTC console from BK tower along with its original code system, and now appears to be in the process of implementing as much field functionality as possible "in relay".

Of course it would be great if more publically accessible museums were able to step up to the plate with functional period correct signaling equipment, but the signal enthusiast community, powered by recent advances in the "maker" scene and high capacity pickup trucks, seem to be taking up a lot of the slack. Just like we have seen with the extensive telecom switch collection of Step by Step Phil, these private efforts stand a good chance of eventually finding their way into durable preservation.

Attempted CTC at the Seashore Trolley Museum

I will also mention that non-FRA regulated streetcar/transit museums have the best capacity to implement "live" historic signaling. When visiting Shoreline Trolley Museum I was shown their "in-progress" CTC system that they were building from thousands of relays donated by Amtrak and Metro North.

 

The fact that I have not heard much if anything about that project in the past 20 years hints at its priority in the greater scheme of things, but all it takes is the right team of enthusiasts to get a project off the ground.

New Signals Up At THORN

 Although not much has changed regarding the ultimate fate of THORN tower, recent photos do show what the new signaling situation is going to look like. In the eastbound direction the triple signal bridge covering tracks 1, 2 and 5 Running, have gotten the Safetrain upgrade with Rule 562 'C' boards turned. Although not yet in photos, it is expected that track 4 will also be getting a PL mast signal to replace the existing reverse direction dwarf. SEPTA is tentatively resuming service to Coatesville upon the completion of new station facilities in 2025 which includes a semi-restoration of #2 track to a point just past the new station for the daily NS regional freight. 

In the westbound direction new PL masts are up on tracks 1 and 2 to replace the reverse direction dwarfs and its assumed that a new gantry mounted signal will be in store for #4 track.  The real surprise is what looked to be a new westbound full PL mast signal on what used to be the Low Grade Freight Line track. The old THORN had maintained a vestigial signal in that same location, but despite the rationalization of the former 6-track full crossover, it seems the low grade stub track has not only been retained, but is being prepared for use. What that use might be remains to be seen, but a mast capable of displaying Medium and Limited speed signals could imply SEPTA trains making some sort of switchback maneuver after discharging on Thorndale's track #4 platform or coming out of some yet to be built storage tracks on the footprint of the old PRR Thorndale facility. 

The THORN interlocking cutover is still planned for later in June with DOWNS to lose its switches before complete removal. GLEN and FRAZER will be turned over to the dispatcher by the end of 2025 closing THORN as an active interlocking station.

NECR / Central Vermont Changes You May Have Missed

While I have been posting much about the loss of former Boston and Maine searchlight signaling on the New Englande Centrale, a couple of other changes on the Central Vermont end of the CT Valley corridor caught my eye. While B&M ownership and CTC ended at Windsor, at some point in the 1990's the Statge of Vermont and/or Amtrak got some money to install about 10 miles of new CTC between there and White River Jct. Unlike the low cost B&M system, the CTC extension had regular 2-3 mile long blocks with vertical color lights, but no new interlockings or passing points.



Upon closer inspection one might notice that the type of color light signal are the short lived mono-housing GRS variety that are also present around LA Union Station, Dallas Union Station and the extreme northern end of the Conrail Lehigh Line.



Anyway in an absolutely baffling decision that was probably the result of generous Federal funding, the NECR also replaced the completely modern CTC extension several years before the 1950's B&M signaling. The new signals consist of hooded LED modular units from Progress Rail The new mast-on-relay-hut signals replaced the older and slightly smaller mast-on-relay-hut signals and feature Canadian style "R" boards in both directions as well as new number places with direction suffixes. The change took place between 2022 and 2024 and mirrors the new signaling installed between EAST NORTHFIELD and Brattleboro, VT.



Another change came in the form of a further CTC extension all the way through the White River Junction area sometime in the 2010's. This change took place between 2012 and 2019 and includes interlockings north and south of White River Jct.



Stay tuned for at least one additional post covering the recent history of signaling in the Connecticut River Valley.

CSX Continues to Attack the Big Four

Turns out that the River Line isn't the only part of Conrail territory currently under attack by CSX signaling crews as I just saw that CSX had replaced the 1990's Conrail vintage signals at CP-37 on the former Indianapolis Line in Wellington, Ohio. This portion of the line had been re-signaled late in the Conrail era and could otherwise be considered "modern" instead of a NY Central legacy holdover. The three track eastbound signal bridge could be considered a local railfan landmark.

The westbound cantilever was likewise replaced. The portion of the Indy Line between Berea and Greenwich is notable for carrying bulk of CSX traffic from the former Conrail territory in NY State before it switches onto the old B&O route to Chicago via Willard. 

To be determined if this is a wide area project or more hen pecking. The signal aspects being Conrail or Seaboard will be a hint. Other signals south of Greenwitch, like these NYC-looking tri lights at CP-189 in Ansonia, OH, are also being replaced.

Unfortunately this is not a part of the country that I have good access to so hopefully some locals are getting all the necessary pictures.

Mr. Beast Fakes the Trolley Problem

For those of you who don't know, "Mr. Beast" is one of the most popular figures on YouTube mostly appealing to 11-14 year old males with formulaic content that tends to involve giving away large amounts of money in novel ways. To get value from this post you do not need to know who Mr. Beast is and I am not recommending you view his content. In 2024-2025 Mr. Beast produced a 10 episode reality TV show for Amazon Prime titled "Beast Games". While watching an in depth third party review/breakdown of Beast Games, I noticed something in the 7th episode the pulled back the whole reality TV artifice a little farther than normal due to my knowledge of how railway signaling and operating rules actually work and I figured I should share it with the class.

The premise of the 7th episode, entitled "Elimination Train" was to set up a trolley problem scenario with full a size AAR locomotive and rolling stock on a Canadian short line. Chosen players would be placed on a surprisingly realistic fake signal bridge with a prop "lever" situated perpendicular to the track. They would then have to choose to direct the train either towards a high value vehicle they could win or representations of their team members facing elimination. (Note, this isn't a faithful representation of the trolley problem which tries to explore the morality of action vs inaction.) While the overall show suffered from a lack of play testing, they did manage to coax one of each outcome for this event.

Anyway this is where the fakery of reality TV becomes visible to anyone with rail knowledge. The sequence begins with the contestant "choosing" the car over the other contestants with the "lever" placed in the reverse position. However to keep up the drama its made clear that he can still change his mind at any time until the train passes the switch like in the conceptual trolley problem. As the full sized locomotive with one car in tow appears out of a smoke effect and approaches the switch the episode went into overdrive with rapid cuts, fast-mo, slow-mo and insert shots trying to build the suspense to see if the contestant will change the direction of switch.

However in every wide shot where the switch and the locomotive is visible, the points are shown to never move from their initial position, even if an insert shot shows the pints moving or the contestant moving the lever. The scenario is run more than once, in one the points are reverse and stay reverse and in the other they are set normal and stay normal. While this might fool the casual viewer, for the rail-informed watcher will know the fix is in once the locomotive appears and is moving faster than a jog, because there is no way in hell some shortline was going to allow the producers (and by extension the contestant) to actually solve the trolley problem and derail their locomotive. To the show's credit the switch mechanism was never shown in detail and the area around where a hand throw switch stand would be was intentionally hidden from view, however I am not sure how much this ultimately mattered for the viewers given the moving points were only shown in extreme close-up.

Here's what I believe went down in the actual production. First, while technically possible, its highly unlikely that a short line would have rigged up a power assist switch to the prop lever when a hidden grip could easily do the same job. The contestant was given some amount of time to make his choice while literal man behind a curtain followed the position of the lever and used the hand throw stand to align the switch points for the benefit of all involved. Then at some point, possibly even before the locomotive emerged from the smoke the choice was set and the train was allowed to do its thing. All of the drama was then manufactured in the editing room.

That's the irony of the situation. The show theoretically sets up a contestant the chance to solve the trolley problem, but doesn't have the balls (or insurance coverage) to actually allow the contestant to solve the trolley problem. What people don't get about the trolley problem is that the choices you are given aren't the only choices that exist. If one puts the points on center, the trolley derails and comes to safe stop. This is how split point derails work after all. (Conversely if you throw the points after the first truck passes over them, but before the second, you can get the trolley to drift and kill both sets of people tied to the tracks.) The real life trolley problem tries to set up a binary choice to explore an moral dilemma, however the mechanics of the problem allow folks with knowledge and the ability to think outside the box to make a choice where everybody lives. That's the important takeaway. If presented with a no-win scenario, you change the conditions of the test. 

BTW, if you want to see how The Simpsons got an NYC Subway interlocking tower surprisingly right, check out this post here.

Guilford Country Resignaling Updates

With the re-signaling of the former Guilford Rail System/Boston & Maine Western Route Main Line accelerating rapidly, I managed to carry out a multi-day road trip to gauge the situation and take what photos I could. With the re-signaling already complete between the Mass border an the Exeter, NH area I started at Dover, NH and worked my way north to Saco, ME.

My first discovery was that CSX was doing more than just swapping out the signaling and the signal rules. Outside of the MBTA zone the interlockings have all been renamed from the Guilford era CPF-Milepost format to location based names, some of which call back to B&M telegraph cores and others more modern things like road names. Overall this marks an improvement as the CPF names were a result of the crazy Guilford owner kitbashing a 715 mile long main line between Mattawamkeag, ME and Sunbury, PA in 1981. Changing the interlocking names was actually in support of moving away from the Guilford's "Freight Main Line" 0 to 715 mileposts. Unfortunately CSX did not restore the original B&M mileposts from Boston and instead established an MP 0 in Portland Maine that increases as one travels west. Although the chaining was not changed, stone "Boston" mileposts defaced with spray paint instead of reused.

Main track Clear at CPF-243

New signal bridge at Rollingsford Jct

In the Dover area the B&M signaling, including classic signal gantries, is very much on its way out. The holdout signals at CPF-244 in downtown Dover are also going to be removed without replacement due to the reduced need for switching movements that need to occupy the main line. Dover actually represents a bit of a B&M signaling island right now as just over the Main border, double tracking improvements have already ushered in Seaboard signaling all the way through Wells, ME.

Part of these improvements include a new full crossover designated WILLIE west of the Wells, ME Downeaster station. The only details of note is that standard CSX switch procurement is now from Apex and the color light signal modules are now from Progress Rail instead of Safetran.

Milepost 211 searchlight automatic. Note the new CSX milepost number.

The section between Wells and Saco was re-signaled by Guilford in the 1990's with brand new SA searchlights, however new signal mast footings are in place at all of the locations as CSX can't let us have nice things.  CSX has also replaced all of the former Guilford defect detectors with brand new units, which runs counter to their practice after the Conrail merger where CR readouts lingered for over a decade.

Turned signal mast at Saco.

At Saco a turned target type signal awaits an unknown fate. The sale to CSX caught some of the Guilford's own re-signaling projects in mid-stride. If any Federal funds were used as part of Downeaster service improvements, it might be difficult for CSX to scrap this work so we will have to see what they choose to do. 

CPF-LO w/b dwarfs

The corridor between Lowell and Ayer has seen no recent activity on the part of CSX. This might be due to the state of Mass owning all of part of the infrastructure or just the lack of passenger operations bumping down the priority. I am pretty sure the state owns the Northern Route Main Line to the wye junction north of town and possibly to the New Hampshire border and plans for Nashua commuter rail will likely dissuade CSX from making changes the government might ultimately pay for. While I already reported on the re-signaling of Lowell's BY interlocking by MBTA, CPF-LO west of the station is intact, although all connections to MBTA tracks have been cut. 

CPF-307 eb-B&M signal bridge

CPF-307 new w/b searchlight mast.

The ABS signals and CTC siding on the Guilford track between Lowell and Ayer are also intact with each end of the siding featuring an intact B&M signal bridge. CPF-307 even has what appears to be a fairly new 3-head searchlight mast, although it is unclear if this was carried out under CSX or Guilford.

CSX Finds a New Old Off Brand M23

I few years ago I posted an article covering changes to the US&S logo over time as seen on its M3/M23 line of electric point machines. This article also covered off brand copies of the M3 from companies such as Patco Rail Services and Vossloh. CSX in particular had gone all in on the Vossloh "VSM-24" product.

Well it appears that CSX has found a new supplier, kinda.  Apex Rail Automation appears to have taken over Vossloh's VSM-24 production line as the only thing that has changes is the stenciling and the lack of a cast company name on the circuit controller housing.

Of course the other news headline is that CSX is replacing its B&O era GRS Model 5F and 5H machines in B&O territory.  Maybe Vossloh made CSX a offer they couldn't refuse and then realized they then couldn't make a profit. We'll see if Apex has any better luck.

NS Ashville Cantilever is Kaput

I have seen photos of the great Southern RR cantilever signal in downtown Ashville, NC being cut down for scrap. With modern style ABS signals in the flood affected area now turned out of service it appears that Hurricane Helene has done what NS failed to do with a previous FRA petition failed to do and end the heritage signaling on at least part of the former Southern S-Line. 

Let's be clear, the only reason the S-Line still had much of its former Southern RR signaling was because declines in coal traffic had ended through trains on the line and NS was trying to actively discontinue the signaling system. Much of the line was still operating under Rule 271 (aka TWC/ABS) with a lot of the signaling dating from around 1980 in the late Southern era with generally more modern signal structures, but split ABS masts following the right-hand-rule.

Although NS appears to be scrapping the Southern-era signals on the hurricane damaged portions of the line around Ashville and into Tennessee, it is unclear if the FRA will re-visit the abandonment or if some sort of bog standard NS Rule 261 setup will replace it.  Also, the parts of the line not damaged by flooding will probably remain unchanged as the State of NC is making noise about passenger service on the route which will come with Federal $ for a resignaling project.

Harrisburg Power Office Is Now Mostly Up and Running

When we last checked in on the Harrisburg Power Director's office about a year ago in 2024, the Big Board had been fully illuminated and most of the Westinghouse "Visicode" SCADA equipment had been reactivated with the help of modern technology simulating the presence of the field equipment like substations and such. However, because the 1930's PRR electrification projects had been funded with depression era WPA loans, the PRR had to spread the wealth and about half the office used GE "Code Selector" hardware that was more complex in operation and not as well documented. 

Well between March and September 2024, members of the Harrisburg Chapter NRHS managed to locate the documentation for the GE gear (it was sitting on a storage shelf) and were then able to restore most of the GE panels to operation, which is especially useful that represents the bulk of what was still operational on the old PRR Main Line up through 2013. The GE gear brings the added fun of step switches being used to store digits instead of all-relay registers for the Westinghouse part of the office.

As if step-switches weren't enough, between September 2024 and March 2025, the team also reconnected the alarm lamps and bell which further adds to the effect.

Still to be addressed are the metering circuits along with programmed simulation elements that would allow visitors to participate in directing flows of 25hz power to virtual train movements. Still, the Harrisburg power office  is mostly complete in terms of being "a thing that functions as it once did" so watch out for planned opening dates as the chapter is still trying to figure out how to manage regular hours.

Caught on Camera: Double Green Clear

 With the CSX takeover of the former Guilford Rail System empire, there's a real risk that we might see the end of double green clear signal indications in the wild. Most commonly associated with the New York Central, double green clear is an artifact of two position ABS semaphore signals with an upper head of R+G and a lower head of Y+G resulting in the combination of R/Y for Stop and Proceed, G/Y for Approach and G/G for clear. A part of the NORAC set of signal indications, its last common use was in New Englande on the former Boston and Main territory operated by the Guilford Rail System and the MBTA.

As previously covered, while Guilford wasn't perfect it did maintain a bunch of legacy signaling practices, like bracket masts, up into the 21st century. Even in places where new signals were being installed, two head "distant" signals to interlockings would still display G/G as opposed to G/R. In fact you can see it for yourself on Google Street view on the milepost 2xx intermediate signal adjacent to the Main St grade crossing in downtown Saco, Maine. 

Lit for an approaching train we see double green clear displayed on a thoroughly modern target type LED mast signal. In fact I saw this street view while scouting Guilford automatic signal locations on Google Maps and was not only able to locate the normally dark (approach lit) signal, but catch it displaying the coveted indication for a northbound Downeaster train. 

Unfortunately it appears that this location will be replaced by CSX within the year with no chance of it retaining the double green clear. However with 5 round trips a day, its pretty easy for anyone in the area to  catch it for themselves. 

It's Electric! - The Westinghouse Brake and Signal Style L Frame

Every so often I mention the Westinghouse Brake and Saxby Signal Company which was the result of the British Commonwealth's Westinghouse Brake Company purchasing the then dominant UK railway signaling supplier Saxby and Farmer in 1901. For those of you who are unaware, George Westinghouse's claim to fame was the invention of the automatic railroad air brake and he set up corporate entities in both the US and overseas to market his produce. In an early example of corporate synergy, Westinghouse also became involved with railway signaling, either directly producing signaling hardware or owning a signaling subsidiary in many markets. Because there is little sense in reinventing the wheel, the Westinghouse corporate empire would cross-license signaling technology between its various arms. This is how US&S became the supplier of the Improved Saxby and Farmer type mechanical lever frame in the United States and how WB&SS Co would come to sell a power interlocking machine awfully similar to what would become the Model 14 family sold by US&S.

Balham Signalbox Style L Power Frame

As railway signaling moved from purely mechanical to incorporate automatic blocks, electric signals and power operated point machines, interlocking machines effectively became hybrids with both mechanical and electric locking. 19th century mechanical methods would continue to provide the route locking, while magnets and solenoids would lock the levers electrically based on track or switch position circuits. 

Electric locks on a traditional "armstrong" type lever frame.

Since safety critical railway signaling relays were quite expensive, signaling suppliers had an incentive to carry out as much logic as possible in the mechanical layer. Therefore even "power" interlocking machines like the GRS pistol grip style or US&S crank lever style still made use of miniaturized mechanical locking grids. 

Westinghouse owned Union Switch ad Signal had already been the North American licensee of the Improved Saxby and Farmer mechanical locking system and has been using it in its crank style power interlocking machines since the 1890's. In 1901 the newly minted Westinghouse Brake and Saxby Signal Company was suddenly in a position to employ US&S's innovations in Commonwealth countries and came out with its Styles A and B interlocking machines, localized for British signaling tastes.

WB&SS Co Miniature Lever Frame in British Service

Like the crank (ie "Model 14") style interlocking machines in North America, the Westinghouse power frames had three distinct logical components inside the box. First, a mechanical locking grid of the S&F type handling route locking. Second, locking magnets that would lock lever travel based on the state of the electric relay logic. Third, electric contact spindles that would make or brake electric circuits depending on the position of the lever. 

Style K frame with mechanical locking grid.

Style K frame contact spindles.

A route conflict would result in a hard lock as steel bits in the locking grid would physically block movement. An electric problem like points out of correspondence or an occupied track circuit would create a soft lock as the lever was stopped by magnetic latches interacting with a rocker arm on the lever spindle. Anyone who has visited HARRIS tower and its functional Model 14 knows the difference in feel between hard and soft.

Magnetic "soft" locks on a Model 14 machine.

All of this was fine until 1929 when the 23 ton weight of 311 lever London Bridge Style K interlocking machine made Westinghouse Brake and Signal question the sustainability of the technology. Unlike North American pattern machines, the British pattern was less lever efficient, needing more physical levers to carry out the same functions. This not only created a weight problem, but also a size problem as the locking grid dimensions grew literally by the square of the lever count. While American railroads had the space to build beefy towers with wide dimensions, the "early adopter" British railways had harder clearance constraints, especially in major cities where large power frames were most likely to be built. For example the aforementioned London Bridge signal box had to orient its locking bed vertically, making the tower an extra floor taller.

 

London Bridge Signalbox

The solution to this was the Style L frame. Externally the Style L looks exactly the same as the Style K, however internally it is quite different with the mechanical locking grid being completely done away with and replaced by electric locking. the Style L existed as yet another step between the purely mechanical interlockings of the 19th century and the all-relay plants that would ascend in the 1950's. However for anyone wondering why an electrically interlocked plant would even bother with clunky miniature levers, there are some advantages. First it maintains a familiar UI and reuses many of the components that were already in production. Second, it still reduces the number of high cost safety critical railroad relays. The latter is not entirely obvious, but an electro-magnet that unlock a contact spindle that a human then turns, is performing many of the same functions as a vital relay without the same need for precision manufacturing. The website covering Westinghouse power frames has both patent and part information so I'll leave digging down into the fine details of how this electric locking was carried out as an exercise to the reader. 

Style L parts

Style L restored

Style L relay locking logic

The Style L completely displaced the mechanically locked style B and K frames on the main line UK rail network with a burst of new deliveries being made to the British Rail Southern Region between 1948 and 1953. Additional frames were delivered to South Africa up until 1960 with a final BR frame delivered in 1962. An interesting ramification of the Style L all-electric logic was a far greater degree of adaptability than those with mechanical locking. In North America, all railroads (except naturally the PRR) had to hire US&S to perform major modifications to the mechanical locking grid. However the Style L electric locking could be modified without the need for specialized tools and British Rail was able to manufacture quite a few "new" Style L plants using left over components or even spare frames obtained for redundancy purposes during World War 2. Even after retirement from main line service, both heritage and miniature railways have been able to adapt surplus Style L equipment for their own track layouts.

 

The final act of the Westinghouse power frame would play out on London Transport in the form of the mechanically locked Style N and Style V frames as their modest lever counts and subterranean placement didn't trigger the same sort of weight issues seen on the main line railroads. In fact, the last Style V frame, think a Model 14 automated by pneumatic actuators) was installed on LT in 1993! (Which goes to show just how wedded the British were to mechanical technology.) With the closure of Wellington A Signalbox earlier in 2025 and Liverpool Lime St in 2018, the last active Style L outside of preservation might be the signalbox at Maidstone East, which was fittingly the last new built Style L delivered in 1962.