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A Union Swith and signal arrangement with a searchlight signal in the lower position, and what appears to be a '3-light' signal in the top position...does this strike anyone else as peculiar or was this a common practice for Santa Fe?
Santa Fe did a lot of "advance" indications, and is one of the few - if not only - roads with a "flashing green" aspect ahead of high-speed junctions. (Which was a complete anomaly, as signaling theory dictates that the solid indication is always more restrictive than the same color flashing.
Most of the time the bottom signal would be dark, and the three light signal on top would show conditions of the main track ahead. Somewhere ahead of this signal (ahead meaning ahead of the train, behind the camera) is switch. If the diverging route on the switch is chosen by the dispatcher, the upper signal will go to yellow (or red), and lower signal will light up. The color of the lower light will depend on the maximum speed allowed through the switch, and on whether this is the signal right in front of the switch or the signal one block in front of that one. The light could be solid or flashing or either.-edit: basically what C855B posted while I was typing this.
It will ALWAYS stay lit when the "A" head is lit unless there was some oddball practice i dont know about as its a big no no to have ANY kind of dark signal. Holding signals are put in to help trains increase braking distance.
On the bottom head, it will remain red if no route changes will be made at the CP youre heading towards. If you ARE making a diverging route, it will display a green, yellow, or FY depending...
I don't think this was true back then. C855B mentioned that a lower unlit head was legitimate practice on the ATSF, and those aspects appear in old timetables.It seems likely that the lower head in the picture only displays one color, though.