Western Railroad Discussion > Clarification on Approach Medium (BNSF)

Greetings-

I have some questions about the use of Approach Medium on BNSF. According to the Altamont Press Timetable (which, of course, is not an official railroad source, hence by reason for asking), it defines Approach Medium as: "Proceed past next signal not exceeding prescribed speed, prepared to advance on diverging route at next signal at prescribed speed through turnout."

It shows both flashing yellow and double yellow (Y/Y) as Approach Medium aspect.

First Question: Is a Y/Y aspect only used when the train will be proceeding on a diverging route, or can it be used as an Advanced Approach (UP term)? In Scenario 1A, I have shown a Y/Y on the approach to a diverging route. Scenario 1B shows what UP would call an Advanced Approach. Is this also a valid Approach Medium aspect on BNSF?

Attached Images: Scenario 1A, Scenario 1B

Kevin Morgan
Arvada, CO
ColoradoRailfan.com

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Second Question: Can a Y/Y aspect be used at an absolute signal at a switch (facing points)? In Scenario 2A, there are no intermediates. I wouldn't think this scenario could be possible because the Y/Y absolute gives the crew to information about whether or not they are diverging at the first switch. For example, take Scenario 2B. No information on diverging or not diverging. Would Scenario 2C be used instead of Scenario 2A?

After doing some more reading, it sounds like Y/Y/R could be an option (if there were three heads) for Scenario 2A. Likewise, R/Y/Y could be used for Scenario 2B. I think R/Y/Y is Diverging Approach Diverging.

Attached Images: Scenario 2A, Scenario 2B, Scenario 2C

Kevin Morgan
Arvada, CO
ColoradoRailfan.com



Edited 1 time(s). Last edit at 03/14/10 13:16 by ColoradoRailfan.

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Question 3: I'm thinking the answer to question 2 above will also satisfy this question, but just in case. Here, we have two main tracks with a train aligned on the north main through a crossover and then on a diverging route off of the south main.

In Scenario 3A, once again there is no information about the routine at the crossover, so I would think Scenario 3B would be used. And, once again, could R/Y/Y be used at the crossover if there were three heads on the signal?

Much thanks for the clarification on this aspect!!!

Attached Images: Scenario 3A, Scenario 3B

Kevin Morgan
Arvada, CO
ColoradoRailfan.com



Edited 1 time(s). Last edit at 03/14/10 14:31 by ColoradoRailfan.

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One reason there are several seemingly identical options in the signals is that the predecessor lines to BNSF today (and the other RRs too) didn't all use the same aspects for the same things. Thus a Y/Y and Flashing Y/R can mean the same thing.

What you actually see depends on the head configuration at any particular location, and what that head configuration is capable of displaying. In this case, if the electronics allow a head to flash, then flashing Y might be used. If not, Y/Y would work instead.

Your example 1A is, I believe, the most common way in which the Y/Y would be used, saying that the train will diverge at the next signal. That third (right) signal could also show R/G, say if this was single track going to double track with the south track clear for 3 blocks after the divergance. In your example, the train would face a stop at the next signal after the third one.

I don't think you would ever see 1B. A double headed permissive signal would only occur one signal prior to a possible divergance and would not be followed by a single head as in your example. Further, a single head would only be possible one signal before a divergance if the switch at the divergance point was a high speed switch, as the train would not need to slow down as much to diverge. The triple track UP in Nebraska is set up this way.

2A is possible. The Y/Y sets the train up for a possible divergance at the NEXT switch, not the current one, so is valid even at an absolute such as you show.

2B is not valid as it does not show the divergance at the switch it protects. I believe that signal would show R/Flashing Y. A triple head would also be possible here, but I believe three headed signals generally only occur when the two divergance points are quite close together and the second one does not have a signal specific to it, in other words they are part of the same interlocking. For example, you have three mains with crossovers between each set of tracks. You are on the north track. You would have a three head as you enter the interlocking and the second head would say you were to cross to the middle, while the bottom head would say you would cross to the south track, going through two switches. But again, there are probably exceptions.

So having said that, 3A is also invalid, but 3B works. 3B would only have a triple head if the second turnout was not signaled and part of the same interlocking.

I wrote the signaling code in Microsoft Train Sim for the Marias Pass 3.0 route and spent many hours working over scenarios so I could get the code to work. In practice, I had to leave out a couple of possiblities and assume that Y/Y would always mean a divergance was coming, while Flashing Y always meant no divergance, even through the rules would indicate they could be used interchangeably. But I'm not a rail and am interested to hear if my interpretations are correct.

And if your favorite RR uses speed signaling, not route signaling (GCOR), then things look much different.

Much thanks for your thoughts here! You confirmed most of what I had thought. I realize that previous railroads equipment may well limit the options, so I guess I should clarify that I'm interested in the "ideal" situation for BNSF. Say we are talking nothing but new lines (or at least new signal) being built/installed. I'm also interested in GCOR signaling, not speed signaling.


bobs Wrote:
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> 2A is possible. The Y/Y sets the train up for a
> possible divergence at the NEXT switch, not the
> current one, so is valid even at an absolute such
> as you show.

This surprised me a little bit. I would still be concerned that you have two possible routes (one head for each route) and an aspect more favorable that stop in both heads. Again, I may be incorrect here, but I wouldn't expect this to be possible.

You are correct about triple head signals when there are two diverging routes within one interlocking. Along those lines, the top signal head would be for the non-diverging route. Would the middle signal head necessarily represent the first diverging route and the bottom head necessarily represent the second diverging route? Or is there no rule as two which diverging route each of the bottom two heads represents?

Again, thanks for the discussion!

Kevin Morgan
Arvada, CO
ColoradoRailfan.com

I'm not quite sure that I follow everything that was said, but Approach Medium cannot be displayed if the route is set for diverging at that location. Any point where you take the diverging route, red will be on top (and it will be a diverging "x" signal).

If there's a scenario where you will take two diverging routes in a row, say going crossing from single main to Main 2 at CP Alpha and then crossing from Main 2 to Main 3 at CP Baker (i.e, your scenario in 1B), you'd get an approach medium (Y/Y or flashing yellow) at the signal before CP Alpha, then a diverging approach diverging (R/Y/Y) at CP Alpha itself, then a diverging clear (R/G) at CP Baker (assuming that the track beyond CP Baker is clear for three blocks).

In general, you're correct that the flashing yellow aspect of Approach Medium is generally followed by an Approach, and the Y/Y aspect is generally followed by a diverging signal. However, the Approach Medium indication requires that you be prepared to either stay on the current track OR diverge, regardless of whether it gave ya a flashing yellow or a Y/Y aspect. So it's good to be ready in any case. :-)

(Edited to correct name of R/Y/Y signal to "diverging approach diverging" whooops!)



Edited 1 time(s). Last edit at 03/15/10 00:10 by bwb6df.

ColoradoRailfan Wrote:
-------------------------------------------------------
> bobs Wrote:
> --------------------------------------------------
> -----
> > 2A is possible. The Y/Y sets the train up for
> a
> > possible divergence at the NEXT switch, not the
> > current one, so is valid even at an absolute
> such
> > as you show.
>
> This surprised me a little bit. I would still be
> concerned that you have two possible routes (one
> head for each route) and an aspect more favorable
> that stop in both heads. Again, I may be incorrect
> here, but I wouldn't expect this to be possible.

If you're changing tracks then you want a red on top, having a Yellow on top instantly indicates that you're going straight, After that you looks down and noticing you have the yellow on bottom would indicate you'll be diverging at the next signal. This is also why 2B and 3A are not possible; without a red aspect on the top there is no indication the train is switching tracks, so the crew would assume they are proceeding on straight rail and operate accordingly.

Just think of it this way: except on a restricting, whenever you change tracks while operating on "Route Signals" you have to have a red on top.

> You are correct about triple head signals when
> there are two diverging routes within one
> interlocking. Along those lines, the top signal
> head would be for the non-diverging route. Would
> the middle signal head necessarily represent the
> first diverging route and the bottom head
> necessarily represent the second diverging route?
> Or is there no rule as two which diverging route
> each of the bottom two heads represents?

There's no rule on that though it would be nice though if BNSF could make it this way, it's not uncommon to get a Red/Green and cross over two or three times within that Contol Point. The three headed signals are used to indicate what will happen after the train diverges, so if the signal which is to be shown requires two heads then having a red on top and the two headed signal below would need three heads. I believe on BNSF this is only Diverging Approach Diverging, and that itself is most likely a holdout from a predescessor railroad which BNSF will probably phase out eventually.

> Again, thanks for the discussion!

Thanks. I thought that a red was required on top if the train is diverging. However, does that imply that the reverse is not true if the train is not diverging, meaning a red is not required on the bottom? In other words, is scenario 2A a valid scenario?

Kevin Morgan
Arvada, CO
ColoradoRailfan.com

Kevin, if you haven't read it yet, look at Al Krug's write up on signals. He explains how the Y/Y is one exception to top head is always the straight (high speed) route. http://www.alkrug.vcn.com/rrfacts/signals/signals.htm
He has a 1999 BNSF signal chart posted and it shows two others, Y/flashing G and Y/G, which are higher speed divergance advance signals.

I'm also not sure my thoughts on 3 headed signals are correct. I've seen them used as I described (Santa Clara, CA at CP Coast), but there is one other scenario where I think they might be valid. Say you have a single track that has two diverging switches, one right after the other, first one going left, second one going right, to two "new" tracks. I believe the second head could be associated with the first divergance (to the left) and the third head with the second divergance (to the right). So R/R/x would mean you could pass the signal and would take the second opportunity divergance. Hopefully someone else will chime in and tell me if this is correct.



ColoradoRailfan Wrote:
-------------------------------------------------------
> Much thanks for your thoughts here! You confirmed
> most of what I had thought. I realize that
> previous railroads equipment may well limit the
> options, so I guess I should clarify that I'm
> interested in the "ideal" situation for BNSF. Say
> we are talking nothing but new lines (or at least
> new signal) being built/installed. I'm also
> interested in GCOR signaling, not speed
> signaling.
>
>
> bobs Wrote:
> --------------------------------------------------
> -----
> > 2A is possible. The Y/Y sets the train up for
> a
> > possible divergence at the NEXT switch, not the
> > current one, so is valid even at an absolute
> such
> > as you show.
>
> This surprised me a little bit. I would still be
> concerned that you have two possible routes (one
> head for each route) and an aspect more favorable
> that stop in both heads. Again, I may be incorrect
> here, but I wouldn't expect this to be possible.
>
> You are correct about triple head signals when
> there are two diverging routes within one
> interlocking. Along those lines, the top signal
> head would be for the non-diverging route. Would
> the middle signal head necessarily represent the
> first diverging route and the bottom head
> necessarily represent the second diverging route?
> Or is there no rule as two which diverging route
> each of the bottom two heads represents?
>
> Again, thanks for the discussion!



Edited 1 time(s). Last edit at 03/14/10 19:41 by bobs.

ColoradoRailfan Wrote:
-------------------------------------------------------
> Thanks. I thought that a red was required on top
> if the train is diverging. However, does that
> imply that the reverse is not true if the train is
> not diverging, meaning a red is not required on
> the bottom? In other words, is scenario 2A a valid
> scenario?


Scenario 2A is valid for the same reason Scenario 1A is... Red is not required on the bottom at all, in fact on UP many signals do not have a red aspect on the bottom as they're not necessary. If you read that signal description again it doesn't say you'll be diverging until the next signal, therefore it can be assumed when you get a double yellow then you're on straight track UNTIL the next signal, and the turnout protected by the double yellow essentially doesn't exist.

Here's an example of a rather common Signal progression we'll see on our territory, I don't know why but we often get Flashing Yellows (Sometimes multiple) ahead of Double Yellows.

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Jaanfo Wrote:
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> Scenario 2A is valid for the same reason Scenario
> 1A is... Red is not required on the bottom at
> all, in fact on UP many signals do not have a red
> aspect on the bottom as they're not necessary. If
> you read that signal description again it doesn't
> say you'll be diverging until the next signal,
> therefore it can be assumed when you get a double
> yellow then you're on straight track UNTIL the
> next signal, and the turnout protected by the
> double yellow essentially doesn't exist.
>
> Here's an example of a rather common Signal
> progression we'll see on our territory, I don't
> know why but we often get Flashing Yellows
> (Sometimes multiple) ahead of Double Yellows.


Thanks for the clarification! That does make sense and your right with regard to the description of the aspect. I just hadn't heard of this scenario so wasn't sure of its existence!

On UP, flashing yellow is an Advanced Approach. So basically, you get a flashing yellow when the next signal is more favorable that stop, but less favorable than clear. Since a double yellow isn't clear, you would get a flashing yellow preceding the double yellow.

Kevin Morgan
Arvada, CO
ColoradoRailfan.com

ColoradoRailfan Wrote:
-------------------------------------------------------
> Thanks for the clarification! That does make sense
> and your right with regard to the description of
> the aspect. I just hadn't heard of this scenario
> so wasn't sure of its existence!

No problem. Here is another example of a signal progression, where the Flashing Yellow is used to indicate to a train an upcoming Converging movement, then a short time later a Diverging movement. I believe UP uses Approach Diverging to Diverging Clear to indicate Converging movements, just like diverging... At least I've noticed in several locations at the end of a track a single red head on the top to indicate the train was Converging into the other.

> On UP, flashing yellow is an Advanced Approach. So
> basically, you get a flashing yellow when the next
> signal is more favorable that stop, but less
> favorable than clear. Since a double yellow isn't
> clear, you would get a flashing yellow preceding
> the double yellow.


It's funny in that we also get a Flashing Yellow in advance of a Yellow/Green (Diverge at 50 MPH on BNSF), I find it amusing in that this means the signals tell you to slow to 40 MPH in order to receive a signal telling you to slow to 50 MPH.

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Jaanfo Wrote:
-------------------------------------------------------

> No problem. Here is another example of a signal
> progression, where the Flashing Yellow is used to
> indicate to a train an upcoming Converging
> movement, then a short time later a Diverging
> movement. I believe UP uses Approach Diverging to
> Diverging Clear to indicate Converging movements,
> just like diverging... At least I've noticed in
> several locations at the end of a track a single
> red head on the top to indicate the train was
> Converging into the other.
>
> > On UP, flashing yellow is an Advanced Approach.
> So
> > basically, you get a flashing yellow when the
> next
> > signal is more favorable that stop, but less
> > favorable than clear. Since a double yellow
> isn't
> > clear, you would get a flashing yellow
> preceding
> > the double yellow.
>
>
> It's funny in that we also get a Flashing Yellow
> in advance of a Yellow/Green (Diverge at 50 MPH on
> BNSF), I find it amusing in that this means the
> signals tell you to slow to 40 MPH in order to
> receive a signal telling you to slow to 50 MPH.


A double head at a converging point doesn't make sense to me unless it is possible that the following signal could protect a divergance (it doesn't in your example). I know sometimes you see two heads at a convergance, but I'm not sure what the second head adds, other than cost!

bobs Wrote:
-------------------------------------------------------
> A double head at a converging point doesn't make
> sense to me unless it is possible that the
> following signal could protect a divergance (it
> doesn't in your example). I know sometimes you
> see two heads at a convergance, but I'm not sure
> what the second head adds, other than cost!

In the demonstration I just didn't think enough to delete the bottom head... I was cutting and pasting things from the other images rather then making it new. (All fixed)

As noted above, I've noticed that UP has a single-aspect head with a Red Lamp on the top protecting a Converging move (Such as at the end of a siding). Not sure if this is the norm for them as I don't work any UP territory, but I like it.

If you know the pre-merger set up, the signal rules make sense....

On the former BN approach speed was 35 and the "normal" speed of controlled sidings was 35, and approach medium used to be a signal that would prepare you to pass the next signal at 35....so flashing yellow was used for both diverging and straight track when the turnout speed was 35 mph. Double yellow was an "Approach Diverging" (A.D.) on the former BN, and was usually for slower crossovers (commonly 25 mph). On the former ATSF approach speed was 40 and the "normal" speed of controlled sidings was 40, so flashing yellow would be used for both striaght and diverging routes where the turnout speed was 40, but was also used for lower turnout speeds depending on where you were. Double yellow was usually used for slower crossovers (commonly 30), but was still considered "approach medium", because the rule included the phrasing "be prepared to enter diverging route at prescribed speed". Faster speeds had faster signals (yellow/green for the very common 50 mph crossovers). There were/are exceptions to all of the above, like everything else on the rr.

The reason for the oddball signals Diverging Approach Medium (the only 35 mph signal) and D.A.D. (with no such thing as an "A.D.") is that ATSF considered red over flashing yellow to be identical to red over yellow (diverging approach) so "D.A.M." was a BN signal (thus they kept the BN speed (35)), as was D.A.D. (since ATSF didn't use triple head signals). The D.A.M. was actually changed to D.A. on most former ATSF subdivisions until the signal dept. reprogrammed all the signals to conform to the new rules. All the rest of the "merged" signal rules are identical to the former ATSF, except approach speed is down to 30 now.

I have never seen a double yellow in advance of a high yellow, but there are still a lot of places where flashing yellow precedes a low yellow.

I believe this is similar to how SP signals were set up in the Central Valley of California in the old days. SP used to not have bonded rails in sidings, so there was no reliable way to get a track occupancy indication in the sidings. Entrance into the siding at the control point in CTC territory was controlled by a Red over Lunar indication with moves to be made a restrictive speed. The approach signal was a single head that would give a Yellow indication irrespective of whether the Lunar was lit at the control point.

My buddy Brad Hellman would be expert on these sorts of things, especially from twenty or thirty years ago.

Ron

http://www.calcoastrails.com/photos/

wyeng Wrote:
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> Where I worked on BNSF almost all signals 2 miles
> before a siding switch had only a single head.
> Most of our sidings were entered on a Restricted
> signal (red over lunar at the switch) so the 2
> mile single head "approach" signal was always
> solid yellow, Approach, if you were lined into the
> siding. A few places had a two head approach
> signal and at those we got a yellow over lunar,
> Approach Restricted, if lined into the siding at
> the next signal.
>
> However we had two places where we entered the
> sidings on a red over yellow or red over green.
> There was only a single headed signal 2 miles from
> the siding switch. If that signal was flashing
> yellow the siding switch could be lined either for
> straight ahead or for diverging onto the siding.
> You did not know which until you rounded the curve
> just prior to the switch and could see the two
> headed signal at the switch. Because the siding
> speeds were 25 mph you had to always slow down to
> near 25 mph approaching the siding switch if you
> had a flashing yellow (Approach Medium) two miles
> out on the single head until you could see the
> home signal at the switch.
>
> We also had a single head high signal at the
> leaving ends of every siding. Thus leaving a
> siding by trailing through the diverging route of
> the switch we got a single green (Clear) at every
> siding we used. You can get a Clear signal at any
> diverging switch where the only choice is to
> diverge, ie trailing.
>
> We also had a junction like that. The first switch
> was trailing thru the diverging route, no other
> choice. Beyond this jct switch was a crossover. If
> you were crossing over you got a red over green
> (Diverging Clear) immediately after which you
> trailed thru the diverging route of the jct switch
> then took the diverging route of the facing point
> XO switch then trailed thru the diverging route of
> the XO 2nd switch. But if you were entering the
> siding there you got a green over red (Clear) or
> yellow over red or flashing yellow over red.
> Immediately past the signal you trailed thru the
> diverging route of the jct switch as above but
> this time you took the straight route of the XO
> which put you onto the siding. For a decade or
> longer if we were lined into the siding at the
> junction and thus had a Clear or Approach at the
> home signal, then we would get a Clear (single
> green) 2 miles before, even tho we would be taking
> the trailing diverging route of the 35 mph
> junction switch and then go into the siding. If we
> were going to go up the main then we'd get a
> flashing yellow (Approach Medium) two miles before
> the red over green home signal at the junction.
> They eventually changed that scenario to give a
> flashing yellow two miles out regardless of which
> way you were going to proceed at the junction
> crossover so crews no longer got a Clear
> approaching the diverging route of the jct switch.

bobs Wrote:
-------------------------------------------------------
> A double head at a converging point doesn't make
> sense to me unless it is possible that the
> following signal could protect a divergance (it
> doesn't in your example). I know sometimes you
> see two heads at a convergance, but I'm not sure
> what the second head adds, other than cost!


I know we have been talking BNSF in this thread, but you will see two heads at a converging point on Union Pacific (without a diverging route on the opposite side) as Jaanfo noted above.

The best example I have of this is at C&S Junction on UP's Moffat Tunnel Sub:
http://www.coloradorailfan.com/gallery/photo.asp?id=102551

In this shot, from left to right, you have the North Yard Siding, Main 3, Bypass 2, and Bypass 1. The siding has a single head because the max authorized speed on the siding is less than (or equal to) the max speed through the turnout. Main 3 and Bypass 1 have two heads because the max authorized speed on those tracks is greater than the max speed through the turnout. Bypass 2 has a single head because it is on the non-diverging route.

So, in this scenario, UP has a "diverging clear" on Main 3 and Bypass 1 when heading westbound to slow trains through the turnout, even though they are converging instead of diverging. Note that the upper head on Main 3 and Bypass 1 is fixed red.

Another interesting fact about C&S Junction. Eastbound trains are on a single track main approaching C&S. Upon reaching the junction, the train may not diverge (Bypass 2), or it may diverge one of three ways (Bypass 1, Main 3, Siding). The signal for eastbound only has two heads though. If the upper head is more favorable that red (G/R, Y/R, FY/R), the train will be on Main 2. If the lower head is more favorable than red (R/G, R/Y, R/FY), the train will be on any one of the three diverging routes, but the signal alone does not tell the train which of the three diverging routes it will be on! I've always though that was kind of interesting...

Kevin Morgan
Arvada, CO
ColoradoRailfan.com



Edited 2 time(s). Last edit at 03/15/10 07:29 by ColoradoRailfan.

Jaanfo Wrote:
-------------------------------------------------------
> I believe UP uses Approach Diverging to
> Diverging Clear to indicate Converging movements,
> just like diverging... At least I've noticed in
> several locations at the end of a track a single
> red head on the top to indicate the train was
> Converging into the other.

Yep, you are exactly correct! Note my C&S Junction example above. I know UP signaling very well. Double yellow is an Approach Diverging aspect (prepared to proceed on diverging route at next signal, not exceeding prescribed speed through turnout, etc). Flashing yellow is an Advanced Approach aspect (next signal is more favorable than red, but less favorable that clear). I just wasn't sure about BNSF and double yellow specifically. Thanks to everyone for the clarifications/examples.

My notes about Approach Diverging and Advanced Approach on UP above is of course in territories whose signaling supports these aspects. Older territories may not. UP also uses flashing red at all control points (at least on the Moffat Tunnel Sub) as a restricting signal.


> It's funny in that we also get a Flashing Yellow
> in advance of a Yellow/Green (Diverge at 50 MPH on
> BNSF), I find it amusing in that this means the
> signals tell you to slow to 40 MPH in order to
> receive a signal telling you to slow to 50 MPH.

That is pretty funny.

Kevin Morgan
Arvada, CO
ColoradoRailfan.com

ColoradoRailfan Wrote:
-------------------------------------------------------
> In this shot, from left to right, you have the
> North Yard Siding, Main 3, Bypass 2, and Bypass 1.
> The siding has a single head because the max
> authorized speed on the siding is less than (or
> equal to) the max speed through the turnout. Main
> 3 and Bypass 1 have two heads because the max
> authorized speed on those tracks is greater than
> the max speed through the turnout. Bypass 2 has a
> single head because it is on the non-diverging
> route.

The number of heads on a signal mast has no bearing on the maximum permitted speed through a siding or turnout. That's up to the crew to know and prepare for.

David.Curlee Wrote:
-------------------------------------------------------
> The number of heads on a signal mast has no
> bearing on the maximum permitted speed through a
> siding or turnout. That's up to the crew to know
> and prepare for.


It does in Speed Signal Territory (Caltrain and many East Coast Railroads), but out here in the wild west they use routing signals and expect the crews to know the turnout speed.