Dogbone (dumbbell)

A dogbone is an oval squeezed together in the middle: two end loops, and between them two closely spaced tracks that look like a double-track main line — from above, the shape recalls a bone. As on an ordinary oval, a train always runs round in the same direction: out on one track, back on the other. The end loops are therefore not reversing loops and need no special wiring, even in a two-rail system; hide them, for example in tunnels, and the layout reads like a genuine line from A to B. A crossover between the two tracks changes that: with it, a train can return to its starting track in the opposite direction, so the layout contains a reversing loop and a two-rail system needs reversing-loop wiring. The same applies to a single-track line with a reversing loop at each end (loop-to-loop), which some modellers also call a dogbone: there, each of the two end loops is a true reversing loop.