SP seemed to be hit and miss as to whether they ordered switchers with signal lights or not. They show up on some H12-44, SW8, S12, SW1200 and SW1200 units, becoming standard on the SW1500s. Presumably the earlier units were ordered with signal lights for their initial assignment to the LA Basin or Bay Area where street running and a multitude of crossings were common.

The two most common signal lights were Mars (figure 8 pattern) and Pyle Gyralights ( an uneven conical pattern). Later signal lights are easier to identify, as the single red lights are in square housings for Mars and oval for Gyralight.

Nowadays, we tend to think of adding locomotives to the system fleet. Except for gen-sets and other green designs, most locomotives are basically assigned to system-wide service or very large pools.

Well into fist generation dieselization, many railroads still restricted locomotives to specific assignments, yards or limtied territories. When it came time to order new locos, they tended to think of filling specific assignments. It wasn't that unusual for a railroad to place multiple orders for various models - light switchers, heavy switchers, light road units, 4-axle raod switchers, six-axle-roadswithers, passener service, etc. Options such as multiple unit, ballasting, gearing, signal lights, dynamics, dual control stands, brake systems, etc, were dependent on the intended assigment. Within the same order of a model might be groups with different options.

For example, SP first bought the TR6 demonstrator, then added three more sets. The SW8 units for T&NO did not have dynamics, but all the SP proper units did for light road service. Of these, 4618-4621/1123-1126 (the last 4 within a order of 10) and 4622-4623/1127-1128 were ordered with signal lights. The follwoing SW900's 4624-4633/1170-1179/1190s did not have dynamics but did have signal lights.

Once and a while, an SP shop would add signal lights to a unit. GE 70T SP #5117 was first assigned to Oregon branches, then to the NWP.