Train engines are equipped with three lights, forming a triangle; the primary (top) light is the brightest. Some federal engineer/brainiac types did a study, and they found that humans could look at the triangle of light, watch it grow in size, as it approached them, and they could then determine how fast it was approaching them (relative, not in MPH/KPH, of course). This was then installed on locomotives as standard equipment, by US federal law, established in 1996.
Here are some key excerpts from their published study, available here (PDF):
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Use of auxiliary alerting lights was expected to have only minimal impact on the workload of train operators because activation of the lights could be automated with tie-ins to other activation tasks. [My headlight modulator is hard-wired, and auto-magically turns itself on/off, as the light level varies between night and day; my two auxilliarly lights are hardwired into a switched circuit, so when my headlight is on, they are on, I never have to worry about them.]
The results of the controlled field tests indicated that the triangular lighting pattern used with each type of auxiliary lights (crossing, ditch, and strobe) increased detectability of the locomotive compared with the use of the standard headlights alone. Each system provided a distinctive, uniform light pattern that motorists could recognize as signifying a locomotive.
The crossing light system provided the best overall performance. The increase in detection distance provided by the crossing light system (over that of the ditch and strobe systems and the standard headlights alone) was statistically significant, providing a greater safety margin.
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This quote is pulled from pages 3-4, of the PDF file (4 pages only). Note that the “crossing lights”, shown in Figure 2, page 3, are the two inside, forward-facing lights on the locomotive, which corresponds to two, forward-facing lights on a motorcycle.
The federal report states: The ditch lights were pointed outward at 15º from the centerline of the locomotive, and the crossing lights were pointed at 0º. [NOTE: the 'crossing lights' were the most effective, in this controlled study.]
Amazing what you can find on the Internet. Google is your friend. Cheers!
Thank you, for doing that bit of research sgt.slag, interesting reading. ...now where can I order some locomotive quality lights 
..........................tony 
