This is a very curious add-on which was quite popular in the past, but is pretty rare now.

This type of light looks quite alot like a single-body traffic light, with four 8" lights, designed to light up when the signal was active and spelling the word "STOP", one letter per light. This was done by placing stencils between the light/reflector and the lens.

The downfall of this type of light was that it put a pretty big drainage of the backup batteries when the crossing was operating on battery power (one of these signals consumes the equivalent to 8 normal lights), and therefore almost all of them were removed.

Some of these still exist in the U.S. (one crossing in Newton, KS; and two in Vancouver, WA). Some Argentinian crossings (the Airport crossing in Rosario) also have these still in service.

STOP lights were manufactured by General Railway Signal, and Union Switch & Signal. G.R.S.A. also manufactured these for Argentina, but their model spelled out the word "PARE" ("STOP" in spanish).

These kinds of lights are only found on 8" signals, and not on 12"x20" or 12"x24" signals.