Up until about the time Amtrak was born, on the SP, a now deceased engineer once told me that the rule in the SP Rule Book when he hired on in the late 1960's was that the White Gyralight was only required to be on when approaching and passing over road crossings at grade. Otherwise, he said the rule did not require you to turn it on. He also said that the rule at the time read that when a set of light engines were run, the Red Gyralight was to be displayed/turned on at all times on the rear unit act as a marker to indicate that this was the rear unit, per SP rules. So to answer one man's question in a post above, yes, you could manually turn on or off the red one as well. Sometime in the early 1970s, this was changed and the rule required the white one to be on while the train was either moving or on the main track (you would have to read the rule itself for all of the excpetions), except you would turn off the white one when meeting another train and/or looking for signals from employees near the track inspecting that train. The red one could no longer be used as a marker light, the classification lights were modified to include a red lense that were then lit up on the rear unit to indicate the rear unit in a set of light engine moves. Later, when the classification lights were done away with, you would turn on the rear headlight (the steady one) on dim to indicate that this was the rear of light engine moves, now with DPU, the rear unit(s) on the rear of a train to indicate the rear of train. A mechanical electrician once told me that there were as many as 18 wires to play with when working with a Gyralight as opposed to 2 wires with ditch lights, or so that is what he said. This was part of the reason, that if the railroads were forced to use ditch lights, then it was ecconomical to use them and retire the Gyralights for this reason (and there were many other reasons, of course as well.)