A
mars light is a lamp that
pulsates or
oscillates to make it more noticeable. The name comes from the Mars Signal Light Company of
Chicago,
Illinois who patented the design in 1935. Yes, they did exploit their name, with some units being fitted with plates reading 'the light from
Mars'; quite suitably given the eerie, pulsing red glow. Similar devices were made by other companies such as Gyralight, but the Mars name has stuck.
Mars lights were mounted on fire engines, police cars and ambulances, but it was on locomotives that they are most famous and beloved. A major problem for railroads in North America was that much of their trackage passed through the wilderness, or across ungated level crossings. Animals would be dazzled by normal headlights, while humans might not be able to perceive the distance, position or speed of a train purely from its headlight. The motion of the mars light makes this far easier.
There are several types of mars light. Some feature multiple bulbs and spin, powered by a small electric motor. Others have multiple bulbs designed to illuminate in all directions ahead, broadening the field of light ahead of the train. Some have a pulsating effect, varying their intensity. Finally, some could change colour - the light glowing red when the train was stopped or undergoing emergency braking.
The lights followed a variety of patterns - the originals moved in a figure-8, but later models moved in an ellipse. The lights could be controlled by the driver to shine facing ahead and supplement the main headlight.
Mars lights have sadly been made obsolete by simpler, more reliable light designs like the rotating dome light, and the pulsing strobe light. Locomotives in North America now mount two alternately flashing white strobes, mounted on the left and right sides of the front (also known as crossing lights or ditch lights). I'm not a fan of the strobe when it's mounted on road vehicles - it doesn't grab your attention nearly as well as the dome light. These new methods are sometimes called 'Mars lights', be it in homage or ignorance.
You can still see the mars light concept in the modern world. If you own the newer models of iBook, you'll have a pulsing white light on the front when the system is in sleep mode. You can also see the effect in many first person shooters. If there's a light in your house with a dimmer knob, quickly turn it between minimum and maximum brightness. Or, simply rent an old gangster/crime movie and look at what's mounted on the police cars. This might also be where the confusion CamTarn mentions comes from: the lights were mounted in chrome casings very similar to the ones that held the sirens.
Sources:
http://www.trainweb.org/gyra/warnlts.htm
http://modena.intergate.ca/personal/pl8s/E!MF49.htm
http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=000REI