ATSF and BN rules, we can see what actually happened.
|BN Chart is a PDF so click the link.|
|Approach Medium (Diverging)|
|Approach Medium (to Stop)|
|BNSF has been investing in 6 lamp distant signals that can display Y/G and Y/Y.|
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|
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.