In Model Trains
In 1906, the Lionel electric trains became the first model trains to use a third rail to power the locomotive. Lionel track uses a third rail in the center, while the two outer rails are electrically connected together. This solved the problem two-rail model trains have when the track is arranged to loop back on itself, as ordinarily this causes a short-circuit. (Even if the loop was gapped, the locomotive would create a short and stop as it crossed the gaps.) Lionel electric trains also operate on alternating current. The use of alternating current means that a Lionel locomotive cannot be reversed by changing polarity; instead, the locomotive sequences among several states (forward, neutral, backward, for example) each time it is started. Märklin three-rail trains use a short spike of DC voltage to reverse a relay within the locomotive while it is stopped. Märklin's track does not have an actual third rail; instead, a series of short pins provide the current, taken up by a long "shoe" under the engine. This shoe is long enough to always be in contact with several pins. This is known as the stud contact system and has certain advantages when used on outdoor model railway systems. The ski collector rubs over the studs and thus inherently self cleans. When both track rails are used for the return in parallel there is much less chance of current interruption due to dirt on the line.
Modern model train sets today use only two rails. Many supply locomotives with direct current (DC) where the voltage and polarity of the current controls the speed and direction of the DC motor in the train. A growing exception is Digital Command Control (DCC), where bi-polar DC is delivered to the rails at a constant voltage, along with digital signals that are decoded within the locomotive. The bi-polar DC carries digital information to indicate the instruction and the locomotive that is being commanded when multiple locomotives are present on the same track.
Some model railroads realistically mimic the third rail configurations of their full-sized counterparts; such models may or may not actually draw power from the third rail (most do not).
Read more about this topic: Third Rail
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