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A Mars signal light (made by the same Chicago family that invented and sold the Mars candy bar, BTW) is two lights (usually one white and one red, but two white was also an option) in a housing. The figure-8 sweep of the light beam is created by mounting the
rear of the housing on offset cams driven by a motor. The front stays relatively steady while the rear goes around in an orbiting manner that makes the light beam oscillate.
The "figure 8" is made by having the housing make two vertical oscillations for every horizontal oscillation. Mars' main competitor, Pyle-National, made a similar unit called a "Gyralight." Its main difference is the motor drive system is simplified, rotating
the unit around a single cam resulting in a circular or elliptical oscillation pattern. Metra is among the last users of Gyralights (indeed, any oscillating lights). They were also very popular on the SP and Rio Grande.
On the 9911A at IRM, there is a control box in the cab that allows the Engineer to not only select the white or red light, but to stop the motor from rotating the housing or "inch" it slowly into a certain position. This allowed the Mars light to be used as
a normal headlight in the event the regular headlight failed. The white light would shut off and the red would automatically display (but keep oscillating) in the event of an emergency brake application to protect the head end of the train until flagging could
be established.
On every U.S. E or F unit I'm aware of, the oscillating headlight was mounted in the upper casing for maximum visibility, with the standard headlight in the lower casing in the nose door. The only use of them outside the U.S. I'm aware of was by Canadian Pacific,
who mounted a twin-beam unit pointed skyward on the roof of some FP7s. There was also a detachable auxiliary light used on the front door of both CN and CP RDC cars that oscillated, but I am not aware of who manufactured it.
Later on in the 60s and 70s, some railroads (like IC, C&O and Rock Island) replaced their motor-driven Mars lights with solid-state units that alternately flashed two sealed-beam lights aimed to cross each other's paths, giving the appearance of the light bean
"sweeping" back and forth across the track. With no moving parts, maintenance costs were reduced. One of the brand names of such a device was Oscitrol.
BTW, pay NO attention to the Wikipedia entry for Mars lights, it is riddled with errors.
Hope this was helpful,
Harold Krewer
Causing More Problems Than He Solves Since 1961
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