A Mars Light refers a form of oscillating signal light made by the Mars Signal Light Company, which the Rio Grande (amongst other roads) used as a secondary headlight on locomotives. Its purpose was to both increase visibility of the train to nearby individuals (pedestrians, cars, etc.), as well as to allow the crew to illuminate things outside the normal headlight pattern.
The Rio Grande initially standardized on Mars signal lights, but transitioned in later years to the Gyralite.
In the cowled passenger units, such as the FT, F3, F5, F7, and F9, the Mars light occupied the upper socket, while the fixed headlight occupied the lower socket. On hooded road diesels so equipped (GP30s, GP35s, GP40s, GP40-2s, SD40T-2s, and SD45s), the Mars light was mounted into the nose.