Woodie:
MARS lights are oscillating lights used (mainly)on locomotives. I'm not sure if many are still in use currently (in the US), but they used to be quite prevalent.
Strobe lights are usually found on top of locomotives and have a very brilliant flash.
I'm not sure about beacons, but I suspect they are basically the same function as strobes, but not as bright.
Ditch lights are lights mounted low on the pilot of a locomotive (or cab car) which either burn steadily, to illuminate the ditches along the right of way, or flash alternately. The later function is usually used when a train is coming into or passing through a station. I've heard that the automatically switch to flashing mode when the bell is activated. Perhaps someone more knowledgeable of prototype practices can fill in more details here. Most DCC decoders can be set up to activate these various lights, in addition to headlights and actually running the locomotive.
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Dave Flinn, Northeast Regional Vice President, NRHS
National Director, Cornell Chapter, NRHS
Life Member NMRA, NER, NFR,
Danbury Railway Museum
[This message has been edited by Dave Flinn (edited 03-30-2001).]