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Friday, August 19, 2016

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.


  1. BNSF does not employ speed signaling. Just because some indications call for speed limits does not imply speed signaling. The name (example) "Approach Medium" also does not imply speed signaling... "medium speed" here does have a specific meaning, but the term is pretty ancient in railroad signaling parlance and has meant many different things.

    In North America, "Speed Signaling" specifically means that diverging aspects indicate definite and transient speed limits. This is contrasted with Route Signaling where the speed limits of specific diverging routes must be published and known prior to the diverging movement. BNSF and its predecessors definitely employ route signaling.

    1. Any time you have signal rules with speed references in the name, it is at least partly a speed signaling system. If BNSF and UP didn't think speed information was necessary, trains would continue to see Y/Y in advance of 50 and 60mph turnouts.

  2. I'm a bit late to this discussion, but I have seen a BNSF signal design document that states

    "Normally the signal for approach to a signal displaying a diverging route aspect will display: Yellow/Lunar for a Restricting signal. Yellow/Yellow for a 25 to 45 MPH turnout. Yellow/Green for a 50 MPH turnout. Yellow/Flashing Green for a 60 MPH turnout.

    "Flashing yellow will no longer be used as an approach to a diverging route. It will be used to provide adequate stopping distance to the second signal in advance. Note: Y/G, Y/FG, and Y/Y aspects should not downgrade to flashing yellow."

    The striking thing to me about the first paragraph is that Y/Y, Y/G, and Y/FG can *all* appear on a distant signal to the absolute signal protecting a facing turnout set in the diverging position.

    The anomaly is that the above-mentioned three distant indications give a clue what the speed of the upcoming divergence is, but the absolute signal protecting the divergence can be any of R/Y, R/FY, R/G and SOLELY depend on the condition of the blocks ahead. The speed of the divergence is not a factor in the colors displayed by the absolute signal at the divergence.

    This made me raise my eyebrows. However, after reflection, it's obvious that the distant signal is where the nature of the speed reduction is most useful. If one reached the absolute signal without a proper understanding of the divergence to be taken, well, it's too late by that point.

    This was a rather interesting bit of knowledge for me to run across; it does not meet the traditional behavior, but it appears to do the job.

    Part of me thinks the western roads should just bite the bullet and go to speed signaling. But it's moot at this point, since PTC is inherently a speed enforcement system.