A common method of dealing with dark signals is the use of light-out protection; wherein an open filament will set the next signal behind to approach. The use of such has become virtually standard, at least in modern computer based signal controllers, and in anything installed since the mid eighties, computerized or not. In the CPU based systems, it comes as standard equipment needing only a simple configuration step to activate it.

Burned out bulbs are detected by sensing a current through the bulbs. Whether lighted or not, at least a tiny current is always passed through the bulb, which is then continuously monitored. If the current goes to zero (filament open), it causes the codes sent through the rails to be set as though the signal were actually red, causing the next wayside signal or cab signal to be set at "Approach" or "Restricting", depending upon how they are configured.

Light out protection on multiple headed signals will also cause the remaining heads to display more restrictive aspects; to prevent an engineer still at some distance from misinterpreting the signal as less restrictive than intended.

Of course, if the bulb opens after the locomotive passes the previous signal, the engineer will still be surprised by a dark signal (unless cab signals are in use).

OPB