The SP light packages included the regular twin-beam headlight, an occilating twin-beam white Mars light or Gyralite and a single-beam occilating red warning light. The twin-beam white Mars/Gyralite has a function similar to that of modern ditch lights. It sweeps across the tracks ahead in a figure-8 pattern, lighting areas the regular headlight doesn't. These lights were common on early cab units of many railroads, like E and F units and PAs, including those of ATSF and UP. There was no requirement to have them; it was just the railroad's choice. UP & ATSF phased them out in the late 50s and 60s but Western Pacific & Rio Grande, like SP, continued using them into the 1980s. As for the single red light, that was a special SP feature. It was a warning light that was rarely used. I believe it was just for emergencies, for a train to indicate to other trains that something was wrong.