Third rail (model railroading)

The use of a third rail in rail transport modelling is a technique that is sometimes applied in order to facilitate easier wiring.

Early toy trains used two metal rail tracks like most real trains. However, manufacturers quickly found that using a center rail for electric power and the two outer rails for common or ground made electrical contact much more reliable and less prone to short circuits. Three rail contact also negates the need for insulated wheels, an important consideration before plastics became widely available. Most scales and gauges that predate HO scale used three rails for electric operation.

A key advantage for three-rail track is reverse loops, where a train enters a loop through a turnout and then exits through the same turnout in order to change the train's direction. With two-rail track, when the track reverses on itself, this causes a short circuit. With three-rail track, because the center rail remains constant and the outer rails are electrically identical, this causes no problems.

The third rail has also been used to automate and animate layouts. An accessory, such as a railway signal, can be wired to a section of track that has had one of its outer rails insulated (not grounded), either at the factory or by a hobbyist. A passing train then grounds the insulated rail, completing the circuit and causing the accessory to operate.

Insulated rails (or rail sections) can also be used to control turnouts, causing the turnout to switch to the position needed by an oncoming train.

Because of this feature, railroad cars intended for three-rail operation will not work on two-rail track unless their wheels are first insulated from each other. Cars intended for two-rail track will operate on three-rail track, but they will not activate controls wired to an insulated rail. Conversion of three-rail cars for two-rail operation, or vice versa, is thus a common practice among hobbyists. It requires either replacing the bogies (wheel assemblies) on the car, or replacing metal axles with axles made of a non-conductive material.

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