Better Know a Signaling System: Staten Island Railway

Now I know what most people are thinking, the Staten Island Railway uses B&O Color Position Light signals and I already covered B&O Signaling years ago.  Well you would be correct on both accounts, however after a recent trip to document the SIR after it's 2004 re-signaling project I decided that the 14-mile long quasi railroad deserved its own special section.  For those of you who don't know, the SIR, sometimes referred to as Staten Island Rapid Transit, was an orphan B&O property that came about due to 19th century efforts to bust one of Cornelius Venderbilt's ferry monopolies.  Electrified for third rail operation in 1925, the B&O made use of the line to test out the new color position light signaling system that the B&O would eventually adopt system wide.  Signaling on the line consisted of ABS operation between the terminal at St George and the other terminal at  Tottenville with interlockings at each end.  A new tower and a GSR NX style interlocking plant was constructed at St George in 1951 while the Tottenville interlocking was presumably under local control until 1990, although I am not sure where the control station would be.

SIR Tower B

Until the re-signaling in 2004, if you were expecting to find main line style B&O CPL masts, one would be disappointed as at some point, probably after the MTA took control in 1971, the high railroad style masts were replaced with CPL dwarf signals on 7 or so foot tall sticks.

The only mast signal was located on track 1 approaching the St George terminal and it was only capable of displaying slow speed indications due to the lack of any orbitals.

While the 2004 re-signaling project fortunately kept the CPL signaling, any though that CTC operation with multiple new crossover interlockings would bring about orbital equipped double sided CPL masts on both tracks was dashed by the incorporation of a new PRR-style pulse code cab signaling system without intermediate wayside signals.  Although the plethora of new interlockings likely increased the total number of CPL signals in service on the SIR, the use of cab signals with ATC allowed the SIR to present a reduced menu of B&O CPL signaling.

Although some high mast and gantry mounted signals were added, orbitals were limited to the 12 and 6 o'clock positions, as cab signals were expected to take up the task of controlling speed approaching the next fixed signal, even in the case where signals were ostensibly back to back.

This limits signal indications to Clear, Approach, Medium Clear, Medium Approach, Slow Clear, Slow Approach, Restricting and Stop.  Limited speed indications are not used and I am unsure as to the use of Stop and Proceed as any wayside signal that can display Stop and Proceed can also display Restricting.



One interesting addition to the B&O CPL rule set is a Flashing yellow / with 12 o'clock marker for what I have been informed is "Approach Proceed Cab", which would replace Approach Medium and Approach Slow and thus saving on the need for extra orbital modules.  I suspect this indication would only be displayed at back-to-back wayside signal locations and could possibly also serve as an Advance Approach as well, which it might be doing in the above situation where a BL20 MoW diesel was making a relay move. Note that while the CPL dwarfs are authentic GRS equipment, the high CPL equipment is long out of production and the SIR opted to use Safetran PL targets with CL-20 modules as the markers.

That about sums it up.  One could simply say that the SIR uses B&O CPLs, minus indications for Approach Medium, Medium Approach Medium, Approach Slow, Medium Approach Slow, but with the addition of Approach Proceed Cab.  However, I am happy to report that after some painstaking research I was able to confirm that there exists at least one 10 o'clock orbital in service on the SIR and it is located at the Tottenville Interlocking 6W signal (west end of the track 2 station platform).

Click to expand!

Used to give train operators on track 2 visual indication of a lined crossover route to head west, the approach medium capable signal was present when it visited in 2003 and remains there to this day as the Tottenville terminal interlocking was not substantially altered by the 2004 resignaling project.  Therefore, despite the slow service and the abysmal headways, I recommend riding all the way to the end of the line where you will be rewarded with what is arguably the most "B&O" CPL on the SIR.  



BTW, the above video proved to be a great resource for the SIR's Tottenville terminal operation.  Skip ahead to 8:30 to see the Approach Medium and 7:30 for an eastbound train approaching Tottenville under a Medium Approach.

Better Know a Signaling System: BNSF Combined Signals

In 1996 the Burlington Northern Railway merged with the Atchison, Topeka and Santa Fe.  Typically when two railroads are involved in a mega-merger the signaling systems are so incompatible that the surviving railroad needs to employ multiple signal aspect systems, or the systems are similar enough that they can be merged without much change.  However when looking at the famous BNSF signal chart we see an almost bizarre mix of route and speed signaling, almost as if two different systems that should have been kept separate were merged.  The shocking truth is that prior to the merger, both BN and ATSF used very similar bizarre signaling systems and the merger was nearly seamless.

Like I said, what stands out is the mix of route and speed signaling.  At home signals it's all route with Diverging Clear and Diverging Approach.  At the distant however we see Approach Medium and Approach Limited, traditionally speed signaling aspects.  This is why it feels like one road was speed signaled and one was route signaled since it would be easy enough to just remove the  speed signal aspects at interlockings and just rely on the timetable while applying speeds to the distant signals. However if one goes back and looks at the original ATSF and BN rules, we can see what actually happened.

BN Chart is a PDF so click the link.

Both these nominally route signaled western roads included speed signaling aspect Approach Medium and Approach Restricting while the ATSF additionally included Approach Limited .   During the unification process BN's Approach Diverging was simply folded into Approach Medium so that the rule now reads "Approach next signal at Medium speed (40mph) AND prepared to proceed on diverging route".  In practice the Y/Y is still used for diverging routes 40mph and below while *Y* tends to be used for 4-block signaling, however this was not always the case on legacy installations, which allowed for two lamp distant masts. 

Approach Medium (Diverging)

Approach Medium (to Stop)

The other signal that stands out is the use of Y/G for Advance Approach.  Now with the Y/Y Advance Approach popular in the east being used for Approach Diverging, it does make some sense to use Y/G, but why was *Y* labeled Approach Medium and then used in the role of of Advance Approach as seen in the preceding video?  Well on BN this remains a bit of a misery, but from the ATSF side of the family tree we got what became the final BNSF rule that states pass next signal at 50mph AND be prepared to advance on diverging route.  It's basically filling the role of Approach Limited, or in Union Pacific parlance, Approach Clear 50. As BNSF re-signals it appears to be common practice for distant signals to display Y/G Advance Approach for Diverging Clear in the 30-50mph range and Y/Y Approach Medium for Approach Diverging (or the extra block length gained with an interlocking allows for a more permissive 4-block signaling). However this could just be BNSF installing signal lamps that aren't used.

BNSF has been investing in 6 lamp distant signals that can display Y/G and Y/Y.

Now I know you must be looking at the signal chart and noticing that there is indeed an Approach Limited using Y/*G*.  Well again, read the rule, not the name.  It's basically the same as Advance Approach, only the speed is raised to 60mph for high speed turnouts.  On UP this is more appropriately called "Approach Clear 60".  

To make things even more confusing BNSF maintains a distinction between R/*Y* and R/Y/Y.  Whereas R/*Y* is typically used for back-to-back diverging movements, on BNSF it is labeled Diverging Approach Medium and is clearly intended for short signaling distances as there is no mention of proceeding on a diverging route and the speed is limited to 35mph instead of 40.  R/Y/Y is the more straightforward Diverging Approach Diverging, but appears on neither of the predecessor systems so as one might guess R/*Y* is still used for back to back diverging movements where BN traditionalists still hold sway.  I have to assume the R/*Y* Diverging Approach indications on the ATSF were converted to plain R/Y.

Diverging to stop at the second signal over Diverging Approach Medium

While BNSF allows for both Lunar white and *R* Restricting, the *R* is clearly the preferred choice nowadays.  Like UP, BNSF only employs Y/L for Approach Restricting without the obvious Y/*R* option.  However unlike UP there seems no desire to distinguish Yard routes with Lunar and occupied block routes with *R* as I have not encountered any 4-lamp signal heads. (For those who don't know, Approach Restricting provides advanced notice of being routed into a yard or non-signaled siding).



So there are the interesting parts of the BNSF signal aspect system.  Hopefully I'll get some feedback regarding how some of the overlapping indications are utilized out in the field and also some more of the historical context I'm not aware of.  Specifically I was unable to locate either a CBQ or GN signal aspect chart to trace to origin of the odd speed signaling components further.

Better Know a Signaling System: B&O Color Position Lights

Ah, here's the one I know you've been waiting for.  What is arguably the most unique and most advanced signal indication system in the united states and arguably one of the best in the world.  Why is this the case?  First, it was designed from scratch in the 1920's, after the advent of high intensity electric lighting and also after most other railroads had adopted some sort of color light or position light system, allowing the B&O to learn from their example.  Second, it had the strongest theoretical underpinnings of any other signaling system.  Another misconception is that the B&O CPL is in the same family as Amtrak's or N&W's colorized position lights.  In fact the B&O is a class by itself as in those other systems position has a one to one mapping to color while the B&O uses the position of marker lights called "orbitals" to modify the central color-position target.

There are so many neat features about the B&O system that I hardly know where to begin so I might as well just list those that come to mind.

• B&O CPLs use the same signal aspects on dwarf and high signals.
• A B&O CPL will only display the color red when the block ahead is potentially obstructed.
• The B&O CPL system is fail safe in terms of bulb out conditions.
• B&O CPL high signals use larger lamps than other position light systems.

In addition to those "nice to have" features, the B&O CPL system follows an algorithm that is both elegant and simple so this time you won't be seeing me going through all sorts of deviations from the norm and musing on how they happened. 

We start with the central target.  This is divided into 4 possible color-positions, Red ---, Yellow /, Green  | and Lunar \.  If there are two signals to a stop, Green is displayed.  

If there is one signal to a stop, Yellow is displayed 

And if the block ahead is obstructed then Red is displayed. 

Lunar is the same as Red, but is used for yards and non-signaled track.  That's all there is to the central target, no exceptions.

 

Next come the marker lights, also known as orbitals, located around the central target and displaying a plain white or amber color.  The six possible orbitals are referred to in their clockface position, 2 o'clock, 4 o'clock, 6 o'clock, 8 o'clock, 10 o'clock and 12 o'clock.  You can also use 12, 1, 5, 6, 7, and 11.  These modify the central target that provides block information with speed information.  Again, I am going to throw out another list here.

• 12 o'clock - Normal to Normal
• 2 o'clock - Normal to Slow
• 10 o'clock - Normal to Medium
• 4 o'clock - Medium to Slow
• 6 o'clock - Medium to Normal
• 8 o'clock - Medium to Medium
• None - Slow to Slow

Even here there are regular patterns with the upper orbitals indicating Normal speed  at the signal and the lower ones indicating Medium speed.  Slow speed at the signal lacked any lit orbital.

Approach Slow at HB Tower

Medium Approach Slow at BAILEY

Of course nothing starts out perfect and the B&O CPL system did have two minor tweaks.  The first is that the "to slow" 2 and 4 o'clock orbitals were changed from a white lamp to an amber lamp for increased salience as a crew could easily forget which side meant which speed instruction.

Slow Approach Slow actually serving as such at BAILEY

The other rules patch was to cover up perhaps the only flaw that was built into the system.  Initially, Green | without any orbitals indicated Slow Clear, but in the 1960's this was changed to Slow Approach Slow since a bulb out on either the 10 or 2 o'clock could accidentally upgrade an Approach Medium or Approach Slow to Slow Clear.  The role of Slow Clear was then replaced by a flashing Green | with no orbitals lit.

Lit orbital with the direction of traffic

Dark orbital against the flow of traffic

Like the PRR and Amtrak, the B&O had an on again/off again relationship with using marker lights for Stop and Proceed indications.  Most times the 12 o'clock orbital would be lit, but the presence of a number plate made it technically unnecessary.  The one time this distinction had a difference was on CTC territory where signals set against the flow of traffic would not illuminate the orbital, despite the fact a number plate still authorized movement.  This makes me wonder if at some point the orbitals were indeed required to be lit for Stop and Proceed reducing reverse direction automatics to absolute Stop.

Just pretend its flashing

 The final change to the B&O CPL system was the addition of limited speed triangles / flashing 10 and 6 o'clock orbitals for limited speed turnouts in a manner similar to the PRR.

If the ICC had ever mandated railroads adopt a national signaling system I suspect it would have been this one simply because it solves some many of the "problems" pointed out with North American pattern signal rules such as using Red as a placeholder and using different signals on dwarfs and masts.  The B&O CPL was installed new as a matter of policy on CSX up through the early 1990's on former B&O territory, but alas the signal bulb maintainer's union was broken and the dreaded Darth Vaders have been spreading ever since. If you happen to be in a position to catch one of these signals in action make sure you do because they are disappearing quickly.

Better Know a Signaling System: C&O Color Lights

The Chesapeake and Ohio color light signal aspect set is the crazy uncle of eastern US speed signaling,  From a distance they look perfectly respectable, carrying on like all the other railroads, but upon closer inspection some major deviations pop out at you.  One might say that the Chessie System merger was a marriage of two equally odd signaling systems, equally odd in two completely different ways that is.  If B&O CPLs are being purged due to their complexity, the C&O system is meeting the torch simply because it is batshit crazy once it gets a little liquor into it.  Let's cut to the chart and see what I'm talking about.

Like I said, at first glance the C&O looks like a very responsible Northeastern style railroad.  You have your Y/G Approach medium, even Y/Y Approach Slow!  That's a big step up on the New York Central and others with Y/Y for Advance Approach.  

Exit signals invite Approach Slow

 R/R/G for Slow Clear is pretty standard and then R/Y/R for Medium App....  Wait...R/Y/R isn't Medium Approach...the label says Slow Approach.  Hmmm, must by a typo because why would you have a third head available and not use it Slow Approach?  Wait...if R/Y/R isn't Medium Approach what exactly is Medium Approach. 

Oh boy, yeah, not I see the problem.  R/Y/Y is Medium Approach.  It looks like the entire indication was an afterthought, sort of like how the PRR used to diverge trains to stop over Approach until 1956.  Still, if you have a third head to play with  there is no need to see the cost of flasher relays come down.  Makes me wonder if they could have gotten away with R/R/Y for Slow Approach and R/Y/ / for Restricting.  This is one of the few cases where a red lamp is lit to upgrade a dark head and upgrade the signal indication (in this case from Restricting to Slow Approach).  This is also one of the few cases where Restricting is more common operationally than Medium Approach. 

Doing with 8 lamps what other railroads do with 9.

It seems that the C&O was really trying to use a two headed signal system, but ultimately had to abandon the idea.  First, Stop is only R/R as the only time an R lamp is present on the third head is for Slow Approach.  Restricting is R/Y in all cases, no R/R/Y. and if trains diverged to stop over Approach then you avoid the third head in most situations.  Placing a lower maker to upgrade Restricting to Slow Approach would suffice in those situations where the turnout speed was low.  Eventually it seems that someone thought better of this idea, but with R/Y/R already given away so the C&O was forced to scramble.  When limited speed turnouts appeared the C&O had to scramble again by using R/*Y*/Y for Limited Approach.  No wonder the C&O had a thing for exit signals.

Enter interlocking?  Pass a signal.  Exist interlocking?  Pass another signal.

This brings up another quirk.  The C&O placed its Red lamp in the upper position on its upper head and the bottom position on the second or middle head.  This gave a wider spacing between Red lamps, but made the concept of "high" green a bit less applicable.

Low high green?

The combination signals Medium Approach Slow and Medium Approach Medium match what was seen on the Seaboard, NY Central and elsewhere with R/Y/G for M-A-S and R/Y/*G* for M-A-M.  I've explained this before, but it assumed a dwarf siding exit signal displaying Slow speed indications.  Speaking of dwarf signals, why don't we take a look at one.

Bottom placement of the R lens on dwarfs allowed for Y/R Slow Approach with Y being Restricting, avoiding the need to flash Y as seen on NORAC.  R/Y was also a Restricting Option.

Slow Clear could be either G or G/R, but in a bit if clever thinking the C&O went with *G* to upgrade slow speed siding exits to Medium Speed, as opposed to NORAC going with G/*R* and leaving *G* for the less applicable Limited Clear.  The rest of the C&O scheme was all pretty standard with the usual mix of Approach Medium/Limited and Medium/Limited Clear.

Is there a slow speed route?  No, just Medium Approach.

In summary, before flashing signals were accepted, railroads were constantly placed in the bind of how to handle both Slow speed signals AND Restricting.  Some, like the Seaboard, went the Lunar White route and used three heads.  Others, like the Reading and NY Central, skipped Slow Approach on high signals.  The PRR dumped Medium Approach in favor of Slow Approach.  The C&O went with Plan D, sacrificing BOTH Slow and Medium Approach in favor of Restricting.  Yes, the third head is common enough so that Medium Approach isn't rare, but there are many situations where only two heads were provided requiring trains to get an exit signal or simply diverge to stop over Restricting.

It's a real shame there isn't much room in our national rail system for unique and interesting signal systems such as this.  Not only does the C&O suffer from not being as lamp efficient as some others, it is a bit less intuitive, relies on dark signal heads and conflicts with many other more popular systems. Going forward only the Buckingham Branch shortline appears committed to the C&O system, however as they remove the remaining signaled sidings on the C&O Washington route there won't be much of an occasion to show it off.

BTW, I am celebrating a milestone at The Position Light today with my 251st post. 

Better Know a Signaling System: Amtrak "CPLs"

I know what some of you are thinking.  Isn't Amtrak part of NORAC and aren't those colorized position lights part of their signaling system and not Amtrak's?  Technically that is true, but the colorized position light was always Amtrak's baby and aside from a single Conrail trial automatic block location near Columbus Ohio, colorized PRR position lights haven't been adopted by any other railroad.  Another misconception is the entire Color Position Light or CPL label for Amtrak's system.  CPLs are what the B&O came up with as each signal relied on both color and position (of the orbital) to derive its meaning.  Amtrak was aware of this distinction and after slapping a bunch of color filters on amber PRR signals and eliminating the center lamp they christened their invention "Position Color Lights" or PCLs.  I call them Colorized Position Lights (CzPL?) because I find the term Position Color Light to be rather pretentious.

More info here.

Found on Amtrak's electrified system from New Rochelle to Washington, DC and all the way out to Harrisburg, the CzPL is either a long overdue update of the former PRR system or a short sighted overreaction to a problem that never existed.  Position Light colorization began as a response to the Chase, MD wreck based on a finding that the all-amber position lights were not salient enough.  Considering the crew of the Conrail train that missed the signal were all high on pot, I'm not sure color would have done much to rectify the situation. In fact many charged the colorization made the signals far less visible, especially in the poor light conditions the amber PLs were designed to excel in.

Can you see me now?

However use of LED light sources have completely eliminated the visibility problems and now the CzPLs are as good as anything that existed in the amber era.  If you are wondering why Amtrak never just cast its lot with color light signals I have heard it is a combination of wanting to re-use old hardware, to avoid needing crews to get used to a new normal and finally, PLs work better in electrified territory with overhead lines and support structures that can momentarily block out single lamp signals.  From my personal observations this is indeed an issue on the Shore Line with color lights winking in and out of visibility.

LED CzPLs certainly pass the snow test.

The last amber PLs were installed by Amtrak in the mid-1980's on the Hellgate line, at the new PHIL interlocking and at FRAZER interlocking on the Harrisburg Line ~1994.  Apart from those locations amber PLs only survived on the Harrisburg Line and un-resignaled portions of the corridor.

Two masts separated by 50 feet and 50 years

In terms of interesting features the only two anomalies are the use of a central lunar white lamp on the lower head for Stop and Proceed indications, which under NORAC are still distinct from Restricting indications.  This allows the lower head to avoid the gaping blank spot the upper head has to contend with.  Also the flashing green | for Cab Speed was not replicated on amber PLs since I guess the NORAC committee no new interlockings would ever revert back to the amber system, especially not ones with high speed turnouts.

New signals, new RGY colors.

So that's the history of Amtrak's colorized position lights.  At some point I'll  do a "gotta chatm 'em all" post, but I believe I am missing videos of both the flashing green | Cab Speed and the flashing / Advance Approach.  Advance Approach is likely the hardest signal to find seeing as how the PRR preferred using Approach Medium for short signal distances.  However they do show up on the 6-track raceway between UNION and ELMORE on the outside local tracks.

Better Know a Signaling System: PRR

Not really sure what more I have to say about the Pennsylvania Railroad position light signaling system.  It is the name of the blog and what I haven't already covered in surveys and news items, it was technically incorporated into NORAC, a signaling system we have already better known.  So instead of just going down and describing all the PRR signal indications I am going to take a different approach and look at how the PRR position light signal indication evolved from the initial PRR semaphore indications on which they are ostensibly based.  Moreover I will also look at the prototype PRR PL system, known as tombstone signals, as well as later developments such as Limited Speed.

I should first mention that if you are looking for a great technical history on the PRR PL you can find one at railroadsignals.us and there you can learn all about the original 4-lamp position light system, sometimes known as "tombstone signals" due to one of the variations of backing plate.

The 4-lamp PL's were an almost literal translation of the semaphore blade positions into lights.  If you are wondering why the PRR didn't just go with using the all color "night" indications, the reason is because reliable high intensity light sources (and mains power) were not yet available yet.  Searchlights and position lights could make use of low wattage bulbs that could run for long periods even on battery power.

Looking at the chart above we see all of our familiar favorites including / over | approach medium and --- over | Medium Clear, but we also see some interesting deviations from the later system, most significantly --- over / Medium Approach.  Also, "Slow speed" indications were handled by a little two lamp analogue of a subsidiary signal on a third head.  These consisted of a --- over --- over / indication, which I believe is Restricting and a --- over --- over |, which plays the role of a Slow Clear or Slow Approach.

Sorry for the small chart there, but I wanted to show how the 1950 version of the PL system compares with the older semaphore system, still very much still in use at that time at various locations around the vast PRR.  One interesting thing is how indications that involved the shape of the semaphore blade (Caution and Permissive Block) had to make use of the upper head \ indication.  We can also see why the Semaphore system had no Approach Slow as Y/Y was the night indication for Caution. 

Prototype PL's, trying so hard for a third head.

From the 1950 chart we can see how  PRR used an exclusively 2 head Semaphore system, which in turn lead to the 2-head PL system.  The Standard Railroad of the World was not about to go around throwing money at unnecessary signal heads, but as the above picture shows, signal engineers were not unaware that the PRR's use of only two heads presented a few limitations.  With the subsidiary signal available for slow speed movements, --- over / Medium Approach was an obvious choice since it was not available as a semaphore since R/Y was reserved for Restricting. However when the PRR PL system was finalized with 3-lamps and 2 heads, Medium Approach had to go for a combination of Restricting and Slow Approach for slow speed movements. If you are wondering how one diverged to stop, spend some time on the LIRR as they still use the original PRR system where one diverges over Approach after first passing Approach Medium with the theory being Approach Medium gets you down to Medium Speed and Approach requires traveling at Medium Speed.

Also making its appearance in the 1950 version of the PRR PL system was the Limited Speed triangle, which allowed for higher speed turnouts (45mph vs 30mph) without having to spend more than 30 cents on a signaling upgrade.  Limited speed turnouts started seeing wider use in the 1930;s and 40's in the electrified region where the higher performance of electric locomotives and multi-track main lines made efficient crossing over more of a necessity. 

I should also note at this time that the ever stingy PRR applied a backing plate only to the | on the lower head because it was thought that any Engineer approaching a signal at Restricting or Slow Approach, would be going slow enough to see the indication even in bright sunlight.

This finally brings us to the 1956 modification of the PRR signal rules to finally include Medium Approach, first cut from the lineup over 40 years earlier.  What changed was that flashing relays were finally considered reliable enough for use in safety critical signal applications.  This allowed the PRR to overload the lower head / so that the speed would be upgraded to Medium when flashing. This method of Medium Approach would be wholeheartedly adopted by Conrail in color light R/*Y* form as a way to, what else, save money over the three head R/Y/R Medium Approach used by the New York Central and Reading systems,



So the only thing left to go over are those funny signals with the extra marker light below them or \ indications on the upper head, which I will do...in another post that focuses specifically on manual block signaling in the United States.  Sorry :-P 

Better Know a Signaling System - Seaboard

Well last time around we explored the signaling system that ate the Northeast.  Now we can explore the signaling system that is eating the signaling system that ate the Northeast. The Seaboard Coast Line railroad, itself an intermodal amalgam of the Atlantic Coast Line and Seaboard Airline, is the 'S' in CSX and the Seaboard system of signals are those which CSX has chosen to standardize on, brushing away the B&O CPLs, C&O color lights and former Conrail NORAC signals.  The reasons is not really due to any inherent superiority, but has more to do with major flaws in both the C&O and B&O systems and the company headquarters being in Jacksonville, FL and NORAC signals were not invented there.  Note I am not going to call these CSX signals because there is still little that CSX has had to do with them, unlike NORAC, which may have borrowed most of the signal rules, but still put its own stamp on them.

Unlike the other dominant signaling system in the South used by the Southern Railway, Seaboard uses speed signals and today is one of the three major codes of speed signaling code along side NORAC and CROR (Canadian signals).  It shares a lot in common with those other codes including the three head (Normal, Medium, Slow) system and Y/G for Approach Medium.  However Seaboard is defined more by its differences than its similarities.

Seaboard's most defining characteristic is its use of Lunar White and only Lunar White for restricting.  There are a few smaller route signaling systems that have this feature, but ever since the availability of *R*, no other major railroad has relied on just Lunar White. 

Back when the Seaboard adopted Lunar for Restricting, Restricting was a seldom used indication, mostly seen where there were routed leading to unsignaled track like yards.  For normal interlockings the Seaboard did not provide any capability to display a "call-on" and therefore they made do with US&S N-3 and 3-color searchlight heads.  However in modern times it is CSX's policy to provide a restricting capability on all controlled signals.  This in turn leads to the frequent use of 4-lamp signal heads, which were nearly unheard of in the area of searchlights and pre-cast color light heads. 

The use of LW for Restricting of course frees up R/Y and R/R/Y for Medium Approach and Slow Approach respectively.  R/Y/R is also used for Medium Approach and is employed by CSX in areas where conversion from non-Seaboard signaling is under way (Conrail, RF&P, P&LE, Etc) or a third signal head is provided.  Lunar white is the first of many instances where CSX management has said "my way or the highway" when its comes to incorporating the signaling ideas of others.  CSX predecessors NORAC, P&LE, RF&P and the C&O all used lower yellow Restricting, which has the advantage of not needing 4-lamp heads, but I guess the south is always right.

 Now just because one had R/Y for Medium Approach doesn't mean that R/*Y* can go unused.  CSX has decided to fill this slot with Limited Approach, which appears to violate the principle of reducing trains to Medium Speed upon passing an Approach indication



Seaboard's other fateful decision was the continued use of Y/Y advance approach, even where the aforementioned alternate speed signal systems had switched over to Y/Y for Approach Slow.  The real kicker is that CSX does not currently use the *Y* aspect, even as an alternate Advance Approach for NORAC compatibility.  This requires that all "patch jobs" require an extra signal head instead of just a flasher relay.  Now if you look closely at the 1989 CSX signal rules I posted above *Y* is listed an a Y/Y alternative, but the original Seaboard set did not include this option and CSX itself later dropped it as well for reasons unknown.

Y/Y Advance Approach on the Abbeville Sub in Bogart, GA where a new interlocking created short blocks.

Without a two lamp Approach Slow indication like Y/Y, all situations involving Slow Speed must use "three headed monsters" to display the Y/R/G flavor.

Like several other pre-NORAC railroads (and the C&O) the Seaboard used R/Y/G for Medium Approach Slow for reasons involving one block sidings that I have already explained.  However CSX chose not to update R/Y/G to the now more common Medium Approach Medium when that indication was added in the 1990s, instead flashing the lower green up "upgrade" the older indication.  If you jump ahead to 6:17 in the following video you can see a CSX medium Approach Medium leading to a R/Y Medium Approach.



In the theoretically sound Caltrain speed signaling system R/Y/Y was chosen as Medium Approach Slow as it worked off the Y/Y/R approach slow and R/Y/G Medium Approach Medium.  Without Y/Y Approach Slow or R/Y/G Medium Approach Medium, Seaboard used R/Y/Y for Medium Advance Approach.  I suspect this might be the rarest signal indication of the Seaboard set as I have yet to positively identify a high signal location where it is employed.

Note: Do not mistake C&O Medium Approach for Seaboard Medium Advance Approach as both use R/Y/Y, but the former is much more common.  C&O signals will be covered later.

Dwarf signals are fairly typical of a speed signaling system with the major exception of Y being Slow Approach instead of Restricting.  The necessity of lunar has made the 4-light Safetrain Unilens dwarf units very popular with CSX.  Here is one displaying the Y/R variety of Slow Approach.

As one might expect R/Y and R/G are Medium Approach and Medium Clear respectively, the latter being perhaps the one case where Seaboard scores a point over NORAC.   One can tell  if a Seaboard dwarf stack protects a medium or slow speed turnout based on the position of the green lamp relative to the red. R/*Y* is provided for Limited Approach, however because of Y/R/G Approach Slow, there is no dwarf version of that indication available. 

 One might expect dwarf Y/Y to be used to the fairly common Advance Approach indication, but, bafflingly is used for Medium Advance Approach.  Unlike using Medium Approach as a Substitute for the missing dwarf Approach indication, a quirk Seaboard shares with NORAC, using Medium Advance Approach would require trains to slow to medium speed first leaving Approach Medium as the short block speed control alternative.

That's pretty much all there is to the Seaboard system of signaling.  It's a solid set of speed signal aspects, but unfortunately was never able to evolve past some of the depricated practices like Y/Y Advance Approach or R/Y/G Medium Approach Slow.  It's a real shame that CSX didn't use the Conrail merger to, at the very least, (re)adopt *Y* as an alternate Advance Approach.