A blog devoted to explaining the ins and outs of North American railroad signaling, past, present and future. This blog seeks to preserve through photo documentation the great diversity and technical ingenuity of 20th century signaling and interlocking hardware and technology. Related topics cover interlocking towers and railroad communications infrastructure.
Note, due to a web hosting failure some of the photos and links may be unavailable.
For the last post of 2023 I'm going to go through my backlog of searchlight related news items. First up is the Florida East Coast where we are all aware of the re-signaling in the Brightline Zone, but it appears that the remaining searchlights on the north end are also on the way out. The Bayard siding, just south of Jacksonville, had no evidence of signal replacement in Feb 2023, but new masts were in place by September.
Up on the former B&M Main Line east of the CSX zone in Ayer, there was been a good deal of searchlight attrition, but this has been a piecemeal process and at milepost 425 near the VT/NY state line the MP 435 searchlight was still standing as of may 2023.
East of Ayer CSX hasn't quite had enough time to start re-signaling projects as they are still trying to rebuild the connector from Warcester to Ayer. Getting out to the two searchlit interlockings between the Lowell and Fitchburg MBTA zones are high on my list.
Up on the former Montana Rail Link the searchlight replacement is ongoing with new signals up around EAST BOZEMAN.
Unfortunately around 2019-2022 many remaining searchlights on the BNSF San Bernardino Sub lost its ATSF vintage searchlights except for a few around the Riverside Station.
Finally on the UP/BNSF Joint Line south of Denver, an interesting situation has developed where CTC has been added to one of the two former single direction ABS tracks and the other has been left as ABS with no PTC due to a lack of need. This has at least temporarily saved a number of searchlights, including the mast at Milepost 226.8 just of US 85 north of Castle Rock, which has had a replacement hovering over it since 2018.
That's all for now. Remember there's no time like the present to get out and get your photos. I'll be doing a followup piece on re-signaling on the New England Central line in the CT River Valley in a few months.
It's sometimes easy to forget that the PRR position light dwarf signal was also used by PRR ally Norfolk and Western Railway in addition to the regularly sized position lights. Their PL dwarf was even included in the 1959 N&W program to partly colorize their signals resulting in something that gave off definite UK "ground signal" vibes, albeit with the ability to display green.
Although the old N&W has been ravaged by re-signaling projects over the last decade, there are still a number of well preserved lines deep in the Appalachian coal country and Virginia. However the same cannot be said for the N&W CPL dwarf as the N&W had color light dwarf alternatives in its rulebook as early as 1961. Therefore the N&W had a multi-decade head start on replacing its PL dwarfs at the point where NS started to get involved. As of the time of this post, it has been reported that there are only 4 N&W PL dwarf signals remaining in active service at three locations. The first location is Iaeger, WV, the second Coeburn, VA and the third, with two CPL dwarfs, is Cloverdale, VA.
The Iaeger example is immediately adjacent to state route 80 just east of the Tug Fork bridge at IAEGER interlocking, just past the west end of Auville yard on the Pocahontas Division Dry Fork Branch near the village of Hull where the three track N&W position lit full crossover used to be.
The Cloverdale pair are located at the east end of the Cloverdale Branch on the Roanoke District's CLOVERDALE interlocking about 8 miles north of the Roanoke Terminal in Roanoke, VA. The first governs the end of a long industrial track that serves a concrete plant and the second serves as the exit signal for the Cloverdale siding.
The final location is actually an automatic distant signal on the otherwise unsignaled Toms Creek Branch for COEBURN interlocking on the Pocahontas Division Clint Valley District. It is located just off Tate Ave, a few blocks from the VA state route 158 and the central business district. All three locations are in the vicinity of recent re-signaling efforts and can be considered highly endangered as a couple of other known examples, including another isolated automatic distant in North Carolina, were recently lost.
Of course rare signal types have always popped up before and I'm just reporting on the results of a forum discussion so if you are aware of an N&W CPL dwarf that has been missed please let me know in the comments! The first draft of this post actually missed the Coeburn dwarf so thank you Oakley for pointing it out.
Just a heads up that about 24 miles of ABS/TWS (aka Rule 271) on the Union Pacific Julesburg Sub in northern Colorado is being converted to CTC as BNSF takes over maintenance of the entire Bush Sub/Julesburg Sub corridor between Union and Sterling, CO. This line represents one of those dual operating arrangements where as traffic levels dropped, two competing railroads turned their parallel lines into a single joint. In the case of the Bush and Julesburg Subs, the division point was the small railroad location of UNION.
Over the last decade or so BNSF upgraded their side of the line and, as their traffic levels increase, they gained more control and have decided to apply CTC to the Union Pacific portion of bi-directional ABS. Although Union Pacific ABS signaling had eliminated use of the pole line and installed PTC, it also made use of short mile long signal blocks with US&S N type color lights at mileposts 77, 70, 69, 67, 66 and 65 in addition to fairly uncommon late model US&S modular color lights at mileposts 71, 72, For example the CTC interlocking replacing the ABS hand throw siding end at MP 58.8 in Sterling will have its distant at the MP 61 signal location. Union Pacific appears to have already been reducing the block length with 2 mile jump to a new Safetran signal at MP 63. The project will also include new interlockings and the currently hand throw searchlit ABS siding between WE and EE MESSEX and two new intermediate signals at mileposts 79 and 78 between MESSEX and UNION.
It remains to be seen if BNSF will remove/replace all of the old UP intermediates or just those directly adjacent to one of the new interlockings, but for anyone in the area all of the existing signals are easily accessible from public highways including US 6 and County Rd 178.
I recently discovered the results of the FRA's Positive Train Control Passenger Braking Algorithm Enhancement project dated September 2023. As regular readers are aware I have been a frequent critic of the overly conservative braking algorithms used by various PTC systems and vastly under perform the performance capabilities of the rail vehicles. While this FRA study was confined to the entirely wireless ETMS system used outside the northeast, it might still offer meaningful improvements to these systems in general. Long story short don't get your hopes up. In fact the results of this study might make things even worse.
So I encourage everyone to try and work their way through the linked paper (mirror here), but in my quest to add value I will provide a summary of the key points. The study examined four potential algorithm enhancements, Target Approach Management (TAM), specified consist length trains, tuned train types, and adaptive braking. TAM is for low speed (5-10mph) approach to stop signals while the other three are general purpose braking algorithms that better take into consideration various quirks of passenger train and EMU/DMU operation. The study was almost entirely carried out via computer simulation where real world variables like train consist, rail adhesion and brake pipe latency were tested in tens of thousands of combinations to estimate real world performance. There also appeared to be real world tests carried out on a test track.
The first important takeaway is that the performance baseline for defining an "undershoot", as in PTC stops the train too quickly is 500 feet for speeds under 30mph and 1200 feet for speed over 30mph. Under the baseline scenario these limits were exceeded 20-25% of the time. The PTC braking algorithms calculate a stop point probability distribution and then add a safety factor. Higher speeds mean more uncertainty so the typical stop point for 25mph might be a couple hundred feet short of the signal, the stop point for 90mph could be multiple thousands of feet short. This is why PTC forces so much aggressive braking at line speed.
As for the results of the study the good news was that the new TAM algorithm improved low speed undershoots (defined as more than 100 feet) from nearly 50% under the current proprietary EMTS implementation to a fraction of a %. Unfortunately that's all the good news you are going to get as the other "improved" general purpose algorithms blew up undershoots in the simulated runs. Specified Consist undershoots increased between 9 and 17% with some passenger train consists undershooting up to 40% of the time. Tuned Train Type saw mixed results with some equipment having less undershoots and some more. Adaptive showed no change for the commuter type equipment while undershoots for passenger type equipment went up from 20 to 30%. The gain for this drop in performance was an improvement in meeting the stop target from 98 to 99%. The real life tests resulted in a mixed bag of performance gains and losses, however unlike the costless simulations far fewer real life tests could be carried out and under much more limited conditions.
The problem with the study is that it ultimately treats a safety overlay system like autonomous operation. PTC isn't what has to stop a train before it hits something. That's the job of a skilled locomotive engineer. PTC is what should step in when the Engineer is clearly going to be doing something unsafe. Even unsafe conditions rarely lead to physical impacts at which point the crash safety system prevent potential injury or death. PTC preventable accidents of all types were already rare and PTC should eliminate at least 98% of those. Meanwhile poor train performance is pushing riders onto the roads where they'll die in car accidents. Celebrating a 1% improvement in PTC effectiveness at the cost of yet more performance isn't the win the FRA thinks it is. The infuriating part is that over on those highways self-professed self driving cars are blowing throw stop signs left and right and Federal regulators do nothing.
Distant signals, as defined as those that appear in otherwise unsignaled territory to warn train movements of an upcoming absolute signal (ie an interlocking), are seen in two flavors in the North American scene, fixed and dynamic. Many year's ago I discussed the use of dynamic distant signals on some Conrail Shared Assets lines in Southern New Jersey and although fixed distant signals are far more common, dynamic distants aren't remarkably rare. Today, most signaling systems make use of two possible distant signal rules. Approach the next signal prepared to stop and approach the next signal expecting a proceed indication of a type defined by route knowledge. As these appear in unsignaled territory neither of these conveys track occupancy information between itself and the absolute signal. CSX on the other hand defined its distant signals by a marker plate that, like the vomiting emoji, can be applied to any signal.
In practice the CSX App Marker would only be applied to Clear, Approach, Approach Limited, Approach Medium, Approach Slow and Restricted Proceed/Restricting. Basically anything that would normally appear on an automatic signal before an interlocking. Now of course this wouldn't be a "Caught on Camera" post if I didn't find an exceptional example of this system. In Cartersville, GA the unsignaled Cartersville Sub approaches the W&A sub at a wye interlocking designed BOWEN. At some point after 2012 CSX re-signaled the W&A sub and installed a new App marked distant signal right off Sugar Valley Road, seen below.
Unlike the typical dynamic distant with two lamps, yellow and green, this example has 5 lamps including a fixed yellow in the uppermost head. This was because the west apex of the wye was interlocked and could give a slow speed route and a medium speed route. Therefore the Sugar Valley Road distant can display Y/R/R Approach for a stop or restricting, Y/G/R Approach Medium for the medium speed route and Y/R/G Approach Slow for the slow speed route. Of course because it doesn't reflect track occupancy the most restrictive indication is Y/R/R Approach as seen here next to an approaching train movement. Ironically, the approach lit distant signal is indirectly reflecting the occupied nature of the block by being lit so at some level, the App Marker lies 😅.
While driving in a brand new rental Mazda sport wagon in Colorado I noticed something interesting on the instrument cluster.
There was a dynamic speed limit indication accompanied by a red tick on the speedometer indicating the currently detected speed limit. Now this sort of information has been available in Waze for some time, accurately using smart phone grade GPS to pull a road segments speed limit from a crowd sourced database and displaying it on the interface. Using something like Android Auto this could be displayed to he driver in any number of ways.
The cybersecurity implications of this aside, I noticed an interesting Mazda party trick that Waze could not perform and that was the seeming ability to detect temporary speed restrictions, aka work zone speed limits. Waze explicitly rejects trying to track TSR's, even for long term work zones. Although achievable through traditional data collection methods, I was aware that the vehicle was equipped with forward facing cameras for its lane-assist feature (or MCAS) and if on-board systems could keep track of lanes, they could also recognize wayside speed limit signs and display them on cab signal, possibly augmented by a Waze style database.
The current national Class 1 PTC standard, ETMS, relies on wireless data and GPS to track a trains position and compare is, Waze style, to a database of authorized speed limits. The problem this creates are clunky data networks that result in slow setup time and en-route loss of connectivity as well as occasional mass outages that can halt traffic across a railroad's entire network. The preferable solution has been evident in the field of vehicle automation for over a decade now. Computer vision systems just look at wayside signs (and/or signals) the same way the human engineer does. To the extent that wayside signs are vulnerable to impairment computer vision doesn't have to replace the current "Waze" type system, but in case ETMS suffers a failure, the backup solution isn't halt all traffic (or god forbid just let the crews do their job), but fall back to wayside sinage detected by computer vision and enforced by the on board apparatus
In yet more fallout from the NS purchase of the former D&H Main Line, the re-signaling ax has hit the siding in Afton, NY after randomly bouncing around the line between Schenectady and Sunbury, PA. The real shame was that at least the north end of the siding, CPF-587, was on track to stay searchlit under CP ownership.
As of 2014 CPF-587 had two CP type GRA SA searchlight masts on the south end that replaced a D&H style searchlight bracket at some point between then and 2008.
CPF-587 also had a D&H vintage single track cantilever mast at the north end that remained untouched until the summer of 2022(!) when a Canadian Pacific in-house type searchlight mast went up.
It is unclear if this was part of a long term Canadian Pacific plan that NS management later nixed or just something NS scraped together as a temporary measure, but by fall 2023 both the out of service bracket and active cantilevers were gone with Darth Vaders and a new relay hut in place.
Although this was always to be expected and CP was in the midst of its own scattershot re-signaling of the D&H line. It is a major shame to lose one of the best preserved interlockings on the route in so short a space of time. Unfortunately the D&H route is well off the beaten path and I my best efforts have only allowed me to nibble at the ends. If you live in the I-88 corridor please do your best to get some photos at those interlockings that remain.
Typically when one discusses a signal cutover one is cutting new signal logic over to control new or existing signals or interlocking appliances. Sometimes one will cut new signals or interlocking appliances into existing signal logic. However in 2014 when SEPTA wanted to relocate 90TH ST JCT SOUTH interlocking 2000 feet to the north to expand a Conrail SAO industrial siding they took things a step further and relocated the entire signaling bungalow.
Above is a street view of the original location in 2014. Below is the new location in 2015. SEPTA didn't even bother to clean the graffiti off the relay hut.
Part of the penny pinching shown here is due to 90TH ST JCT SOUTH being solely the domain of Conrail freight movements before they hit the SEPTA Airport Line at 90TH ST JCT NORTH. Without a Federal or State grant to pay for the capital improvement, there was no incentive to pad things out and create more jobs and with traffic being pretty minimal there wasn't a cost incentive to minimize down time due to testing. This just goes to show when the costs align, railroads will save money on signaling.
In a completely predicable turn of events, the iconic Southern Pacific cantilever mast that has guarded the southern approach to the 4th and Kin St terminal in San Francisco has met its demise sometime between March and October of 2023, likely in conjunction with the erection of overhead electrification wires. It has been replaced by three LED searchlight dwarfs.
I've noticed that starting with Amtrak's New Haven to Boston electrification. recent North American electrification projects have included far more overhead clutter than those seen in Europe. Specifically the use of solid overhead beams to mount the wire brackets as opposed to cable spans. Cable spans don't tend to block signal sight lines as much, allowing existing signal placements to remain.
Anyway, I'm not sure if the 4TH ST cantilever was scrapped or donated to a museum, but as one of the last of its type in daily service, its loss is significant.
It has been a decade since ALTO
tower in Altoona, PA closed and after a conversation with persons
directly involved in the process I have come to learn that the
likelihood of its preservation seems increasingly remote. The good news
is that there are currently no plans to demolish the tower by Norfolk
Southern, however the previously touted move the tower in its entirety
to the Railroaders Museum about a half mile to the east are unlikely to
ever come to fruition due to the structural condition of ALTO tower itself.
Immediately
after the tower's closure there were two competing plans about how to
preserve it. The first was to move the tower in its entirety in a
similar process to the recent move of WB tower in Brunswick, MD. The
second plan was to gut the tower of all internal components, strip off
all of the architectural ornamentation and then reconstruct the tower
from scratch. One of the advocates for the second plan expressed that
his motivation to sacrifice the 1915 structure was due to it being
fatally compromised by termites and requiring asbestos remediation. All
up, full preservation and relocation of the 1915 structure would cost
multiple times construction of a duplicate fitted out with the internal
components and external ornamentation. It would also avoid the fate of
MO tower about 10 miles down the Main Line in Cresson, PA that literally
fell apart during a late 1990's effort to relocate the structure to a
nearby railfan park due to compromise of the 100 year old wooden
structure.
Nevertheless, higher-ups at the Altoona Railroadsers
Museum decided on the full preservation plan and drew up a phone book
sized document about the tower and the process (with more than a few of
my photos included I might add). At this point the sizable cost of the
preservation plan became a barrier to implementation along with whatever
roadblocks NS would be inclined to throw up. In the following decade
the tower has started to lose interior items to theft with the bank of
glass timers being an early casualty. Although it is more secure than
some of its peers, the wooden structure remains vulnerable to vandalism
and fire. BTW I attempted to gain a PDF digital copy of the preservation
document from the the individual I was speaking to, but he never
followed up after I gave him my contact information. Hopefully he will
become aware of my interest and reach out.
Let me be clear. I am not saying that ALTO
tower is doomed to demolition or fire. It is immediately adjacent to a
major roadway and could still be saved according to the 2012 plan. The
point of this report is to provide the community with an
update/explanation after the well known preservation plans seemed to go
nowhere. I don't have independent verification of the claims expressed
to me or independent analysis regarding the best course of action. The
better option is absolutely relocation of the 1915 structure, but we
should all be clear eyed about that not being possible. It's also
unfortunate that the ideal option of preserving ALTO in situ appears to have never been on the table due to NS and liability paranoia.
While trying to gather information for my recent SEPTA Unilens post, I discovered a newish railfan video channel with a large amount of up to date of front facing video content that can help with signaling research. Retired Railfan Horn Guy has been crisscrossing the country shooting a mix of front window and standard videos with special emphasis on SEPTA and, more recently, Caltrain. The latter is particularly useful given the impending demise of forward facing views on that line.
The guy is based on Long Island so naturally there is LIRR content, but he doesn't seem to have the same level of access as some of the other LIRR specific video channels that I suspect have insider access. Anyway, its a fantastic channel and good resource to bookmark.
In yet another chapter of the rail industry's love-hate relationship with the Safetran Unilens signal, over the last few years SEPTA has been systemically replacing its Unilens signals with L&W LED searchlights similar to those being employed by Amtrak in the Chicago, New Orleans and Albany terminal areas. Although I am in the Philly area and perform various SEPTA excursions I only just noticed the change since they were limited to the former Reading Company lines I haven't ridden lately. Below is an example of a compact Unilens "high" mast at JENKIN interlocking since 2006 that was converted from Unilens to LED searchlight at some point over the last 2 years.
Track 2 LED searchlight high dwarf at JENKIN in 2003
Track 2 LED searchlight high dwarf at JENKIN in 2010
Combing through some SEPTA territory railfan window videos I can confirm that the majority of Unilens signals have now been replaced by LED searchlights. including those at NEWTOWN JCT, TABOR JCT, the Fox Chase sidings and Norristown Elm St. Because of SEPTA's use of reduced aspect signals, new searchlights won't get much opportunity to demonstrate their full rage of colors and the clipped flashing might be a little irritating while displaying "cab speed" indications.
LED Searchlights at Albany Union Station
For those of you who might be encountering the Unilens saga for the first time, the Unilens was created as a solid state replacement for the traditional electro-mechanical searchlight. It makes use of plastic light pipes (think big fiber optics) to deliver light from up to 4 lamps to a single lens assembly. While this might sound like a great solution, apparently the product suffers from expensive propritary incandescent bulbs and degradation of the light pipe that reduces output over time as well as giving green indications a yellow cast. While some of SEPTA's Unilens dwarfs were approaching 20 years in service, some, like those at Norristown, had been installed less than 10 years ago. It's entirely possible that the long term costs of operating the type became just too much. It will be interesting to see if LED searchlight technology migrates to the Class 1's, which still seem wedded to the Unilens for restricted clearance applications.
The typical view of Pennsylvania Railroad position light signals mounted on a steel angle iron signal bridge is one where two heads are mounted on a short pole that is in turn attached to the signal bridge structure.
But what if I were to turn this view...about its vertical axis.
Do you see it yet? How about now...
As is now obvious, the original PRR concept for mounting the then new position light signals was to place the lower head on its own little mounting mast, forward of the upper head. In the above example at the late CP-PENN, this provides all around maintainer access to the both signal heads to replace bulbs or clean the lenses, although period signal bridges would have afforded this access only to the upper head. As time went on, the more common single pole mounting became standard. Still, the earlier step-mounted position lights were left in place.
Above we can see the eastbound signals at the late CP-GRAY with an original configuration step mounted PL in the center, a modified step mounted signal with a Safetran lower head on the right and a standard mount PL on the left.This reflects the PRR Main Line 2x2 Rule 251 configuration that was later altered by Conrail to double track Rule 261.
With most of the PRR position lights now removed from Class 1 service, the best place to catch stepped position lights is on Amtrak's Northeast Corridor between Philadelphia and Wilmington. The above example are the northbound signals at HOOK interlocking in Macrus Hook, PA. The stepped signals seen below are the southbound signals at BALDWIN interlocking in Eddystone, PA.
Like the previously discussed compact position light mounting, the stepped mounting is another one of those things you might never have noticed until it was pointed out.
Although I did not get confirmation about where the control was passed to, I suspect that it will follow QUEENS, and NASSAU with the territories being remote to the dispatch center at the Jamaica offices. It is unknown if installed panel or VDU interface was removed or if they remain available for emergency restoration.
In addition to the closure of VALLEY, I was informed that BABYLON and BROOK previously saw their operators relocated to nearby yard offices and DIVIDE was also closed in 2021 in conjunction with the opening of the Main Line Third Track. This means that VALLEY was the last full time, stand alone interlocking tower on the LIRR with LEAD technically being in the bridge cabin format. It seems that the LIRR has just witnesses the same collapse of interlocking stations that SEPTA saw in 2003.
In other news, L&W brand PRR pedestal signals have appeared at ROCKY interlocking on the Babylon Branch, Rocky is the last remaining position lit crossover on the branch and the LIRR uses pedestals as a precursor to re-signaling events, however the move to reduced aspect signals have made this progression less necessary and the use of new peds could be permanent in order to decommission the aging signal gantries.
I also had the opportunity to visit M CABIN that controlled the Main Line Cutoff drawbridge over the Dutch Kills in Queens. Unfortunately the cabin has been gutted of any interlocking hardware by local scrappers.
The LIRR has a number of zombie towers such as this like BLISS and HAROLD and I'll try to get definitive answers on more of them.
When the Pennsylvania Railroad needed a reduced clearance version of their brand new position light signal, they made a position light version of the semaphore dwarf signal. About a decade later when the PRR needed a compact signal that could also display the full range of signal indications they invented the pedestal.
But what if you are worried that pedestals with their pairs of plain white lamps just weren't visible enough in fog or smoke at main line speeds? Well you might need a compact position light.
If one looks at the back of a Union Switch and Signal position light setup, one will see the lamp modules are mounted on the ends of metal tubes extending out from a central tub. The backing plate, if present, is then affixed to the ends if the tubes. However you might realize that there is nothing in this setup that physically requires the lamp modules be mounted where they are mounted. They can be placed at any point along the tube. As long as there is a hole for the wire they could even be placed directly against eachother! Which is exactly what the PRR did when it needed a reduced clearance PL format.
Don't let the square targets fool you. These signals on the LIRR Main Line are completely ordinary US&S position lights, just shoved together. The compact format for the Milepost 7.6 automatics was chosen to be visible underneath an overbuild just south of the Kew Gardens station.
You can see the size difference in comparison with the full size signals on the opposite side of the signal gantry.
While these LIRR square backed compact PL's are now likely unique due to the HAROLD interlocking re-signaling project, compact PLs in general still exist in several others locations with the most accessible being DOCK interlocking in Newark, New Jersey, on Amtrak's NEC.
At the east end of the station on tracks 1 and A, the 90LA and 90LB signals were both in compact formats of varying degrees of compactness. The 90LA on track #1 even had different geometries for the upper and lower head given the unique clearance requirements.
In the late 1960's a new crop of "Space Aged" rapid transit systems began to take shape in North America making use of the new materials and electronics developed since the end of World War 2. In particular was a push to replace wayside signals and trip-stops with cab signals which would enable both automated operation and a reduction in wayside hardware. Of course this forced the question about contingency operations in case of cab signal or related failure. Railroad explorations of wayside elimination in the 1930's and 40's had retained wayside signals at interlockings and other controlled points and rapid transit followed suit with the further innovation of reduced aspect signals displaying Stop and Proceed aspects with the occasional Diverge and Absolute Block. Also like railroads, the rapid transit systems adopted a mix of "dialects" for their reduced aspect signals that I hope to categorize below.
Lunar White - The Granddaddy of them all, the Lunar White proceed
indication can be considered the "default" rapid transit proceed signal
aspect. First appearing on the PATCO Speedline in South Jersey (at least
as far as I can tell), lunar proceed was later adopted by systems
including DC Metro, SEPTA MFL and Route 100, HBLR, Cleveland Rapid
Transit and Baltimore Metro (as built). The rationale was to be distinct
from the existing ABS signal aspects using Green and Yellow and present
as a railroad Restricting style aspect for non-cab signal equipped
movements.
PATCO Lunar White Cab Speed
Steady Green - Considered the "obvious" solution since Green means Go, use of Green in place of Lunar White has been gaining in popularity with newer systems as any perceived need to be distinct from older ABS systems has faded. Notably appearing on DC Metro peer system BART in 1972 it was also adopted by Maimi MetroRail, St Louis Metrolink, Baltimore Light Rail and the Baltimore Subway as modified. Some systems will use flashing green to indicate a diverging route, others yellow.
Baltimore Metro Steady Green Cab Speed
Flashing Green - Taking another page from the railroad playbook, flashing has made a few appearances to indicate a proceed indication on rapid transit systems. On the New York city subway flashing green straight up means Cab Speed for both straight and diverging movements under the control of the CBTC system. On Atlanta's MARTA, flashing green is the default proceed signal with steady green indicating a diverging route.
Yellow - Similar to lunar white, this substitutes lunar for yellow similar to that dialect of railroad Restricting indications. This is most prominently used in Boston with Y/R for straight routes and R/Y for diverging.
MBTA Y/R Cab Speed
Green Arrows - This most prominently appears on the cab signaled portions of the Dallas DART system to differentiate from the ABS signals and avoid the use of flashing.
DART arrow signals, not illuminated.
DART arrow signals, illuminated.
White Arrows - Like the green arrows above, but using either lunar or plain white. This is popular with airport people movers including the JFK AirTrain but also in use on the Sound Transit light rail with stylized direction indicators.
Sound Transit stylized arrow.
ABS - Currently used on Chicago, this method of go-no-go signaling takes a cue from the cab signal state to display a Green if the cab speed is "clear" (55/70mph) and a yellow if it is "restricted" (35/25/15mph). The cab signals can be from either block state or due to civil speed restrictions.
CTA Proceed Clear
CTA Proceed Restricted
This is my best shot at a taxonomy. I'm sure I've forgotten about a few systems or corner cases so if you, the reader, can think of any, please let me know in the comments. Please make sure that the line is actually cab signaled as there are quite a few ABS signaled light and heavy rail transit systems in North America.
