Y/Y vs *Y* Advance ApproachY/Y Adopted By: Seaboard, CSX, New York Central, New Haven, Southern, N&W, Reading
*Y* Adopted By: NORAC (Conrail), CN, CP, IC, UP, UP, Caltrain
This is one of the more common distinctions in the Eastern half of the United States. Advance Approach informs the engineer to be prepared to stop at the second signal. It is used in short block situations (ie less than 2 miles at normal speeds) where a 3-block system using only Approach would not provide sufficient stopping distance. I'm sure I could look up the real history of how this dichotomy developed between using Y/Y and Flashing Yellow for Advance Approach, but I'm just going to turn this into a story between the New York Central, which was a Y/Y road and the PRR, which ultimately became a *Y* road, or at least its successors did.
|You really needed a second head for that? This ain't semaphores.|
Y/Y Advance Approach is one of those old-timey things that have somehow survived into the modern era. It was used back in the days of the semaphore by a few high density railroads that didn't want to overload Approach Medium and didn't want to constrain their engineers. Conrail had the oppurtunity to put it to bed with the formation of NORAC, but unfortunately CSX is a bit more hidebound and has been turning back the clock when re-signaling former Conrail territories.
Y/Y vs Y/G Approach "Diverging"Y/G Adopted By: East of Mississippi and Canada
Y/Y Adopted By:West of Mississippi
Approach "Diverging' signals, either speed or route, are those that instruct the engineer to approach the next signal at a proscribed speed in order to safely take a diverging route. This is an interesting East vs West thing I don't really know where it came from, although I suspect lower quadrant semaphores had a role to play. The east almost universally uses Y/G to instruct engineers that they are about to diverge, while out West Y/Y is the standard. Canada beaks the tie with Y/G, but it still makes me wonder why the western roads wanted to be different. Like I said I suspect it has something to do with lower quadrant semaphores as Y./G is actually severely under utilized in the west, having only been recently resurrected for Approach Limited type indications.
|Route signaling, but Yellow over Green still means diverge.|
Y/Y vs Y/R/G Approach SlowY/R/G Adopted By: New York Central, Seaboard, Reading, NJT,
Y/Y Adopted By: PRR, Caltrain, Amtrak, B&M (Guilford), Conrail, Canada
Here is a Speed Signaling special that certainly doesn't apply anywhere out west as all those guys use Y/Y for Approach Diverging. Another interesting point here is that sensible signal aspect systems like NORAC allow bot BOTH versions of Approach Slow, yet member railroads show distinct preferences for which they tend to adopt. Approach Slow is exactly what it says on the tin. Y/Y is an obvious choice because if Y/G is Approach Medium, downgrading the lower head to Y reduces the speed to Approach Slow. Of course Y/R/G is also an obvious choice since Y/G/R is Approach medium and moving the Green to the lower head downgrades it to slow. Y/R/G is the position theory version and Y/Y the color theory version. For roads that already adopted Y/Y for Advance Approach Y/R/G is what you are left with. Unfortunately this requires the use of so called "three headed monsters" and also precludes Approach Slow from being displayed on dwarfs.
|Whole lot of slashes going on.|
R/Y/G vs R/Y/*G* Medium/Approach MediumY/R/G Adopted By: NORAC, D&H, NS,Caltrain, B&M
Y/R/*G* Adopted By: Seaboard, CSX, NYC, Reading,C&O
Typically trains run on the model of prepare to diverge, diverge, proceed. However when where finf a situation where one needs to diverge, then immediately diverge again you need a special "combination" signal that provides explicit speed/route instruction for the first action and then the next. Now, the majority of railroads have tended to eschew these sorts of signals, either providing interlocking exit signals or allowing trains to diverge over a signal where the immediate action is implied from the preceding signal or over restricts the train for the next signal (think Medium Approach instead of Medium Approach Medium).
|I like to call this one "The Christmas Tree"|
When Conrail and the other Northeast roads created NORAC this anachronism was thrown out and R/Y/G was given over to the now more common Medium Approach Medium. In fact Conrail didn't even bother to adopt any Medium Approach Slow indication. Where sidings were equipped with slow speed exit signals they were upgraded to show Medium Clear using G/*R* from G/R Slow Clear. Again, the reversion of R/Y/G M-A-M to R/Y/*G* on former Conrail territory being changed to Seaboard signaling can be considered a step backwards on CSX's part.
R/Y/Y vs R/*Y*/(R) Diverging Approach DivergingR/Y/Y Adopted By: UP, BNSF
R/*Y*/R Adopted By: Southern, N&W, DRG&W, BNSF
This is the same issue as above only as seen in route signaled systems. One would assume that R/Y/Y would be the default seeing as how Y/Y is the standard Approach Diverging indication. However R/Y/Y requires three heads and when one is upgrading a Restricted Speed siding into a signaled siding it would be nice to not have to modify the two headed searchlight with a lower head that can display Y, R or L. The easy solution for signal engineers in the 1950's with newly reliable flashing relays was to use R/Y for Diverging Approach and R/*Y* for Diverging Approach Diverging.
|BNSF Diverging Approach Medium. It's flashing, honest.|
R/Y/R Medium ApproachR/Y/R Only Adopted By: NYC, Reading, D&H, N&W, Canada, B&M
One expects rare signal indications to require three heads. Approach Slow, Medium Approach Medium and the like aren't the sorts of signals one will see displayed every day. However quite a large number of railroads adopted systems that only allowed Medium Approach to be displayed on three headed masts. to be fair there was a time when almost all home signals were expected to have three places, if for no other reason than to allow for the use of a subsidiary semaphore in the bottom position to display R/R/Y Restricting, but quite a few railroads allowed this practice to persist well into the modern era.
|No flashing needed on three headed signals.|
R/Y vs R/R/Y RestrictingR/Y Adopted By: NYC, NORAC, C&O, N&W, EJE, RF&P, Canada
R/R/Y Adopted By: Southern, Reading, FEC
For concept so common in North American signaling I don't know of anything else with more options to be expressed. From number plates to marker lights to flashing read and lunar white it seems that every railroad had a different way of telling trains to move slowly and not crash into stuff. One of the most interesting distinctions is between railroads that allow R/Y Restricting and Railroads that only allow R/R/Y Restricting. I term these two 'lower yellow Restricting" and "bottom yellow Restricting".
R/R/Y restricting originally came about from very early interlocking technology where the Restricting indication would be controlled via its own lever or would utilize some different system to display since a Restricting was what one would pull up when the signal would not display normally. The early R/R/Y often involved a smaller semaphore flag or ground mounted semaphore called a subsidiary or call-on signal that would only be pulled up when the train was standing at the Stop. If the subsidiary signal was not needed at a particular location the railroad would have the choice of just providing a red marker or just assuming the crew could tell a main signal from a subsidiary signal and then allow R/Y to be used for something else. If the second head wasn't present you would have one main signal with one subsidiary far down on the post.This ultimately resulted in the bottom yellow form of Restricting where two headed signals need a gap between the heads to change a Diverging Approach into Restricting.
|That oddly shaped lower head is the last vestige of the "subsidiary" signal.|
Y vs L or *R* RestrictingR/Y or R/R/Y Adopted By: Most Eastern Roads, C&O, Canada
L or *R* Adopted By: Most Western Roads, Seaboard, B&O, PRR
The final piece of signaling dialect I plan to tackle today is the age old argument between making Restricting the old man out with a strange color or having to flash or by making something else the odd man out by giving Restricting a yellow slot on the lowest head. Incidentally all of the position light systems, ie the PRR, B&O and N&W, all count as being in the second camp as their forward slash would count as lunar in a color light system. This is of course clear on B&O and Amtrak where that slash is also colorized lunar.
|Thanks Seaboard Coast Line, you just had to be different.|
Anyway, I could go on and on with this sort of thing, but these are the major ones and I will be referring to them as i explore the various signal indication systems used in North American in my upcoming series Better Know a Signaling System.