A wye in American railroad terminology, known as a triangle in British terminology, is a triangular shaped arrangement of tracks with a Y-shaped switch at each corner. With a sufficiently long track leading away from each corner, a train of any length can be turned.

Turning is required for any directional piece of railroad equipment, such as most steam locomotives, or indeed many passenger trains, especially those that have a dedicated tail end car such as an observation car.

Individual locomotives and railroad cars can be turned on a turntable, but obviously whole trains cannot. A wye, or a loop, are the only ways of doing that.

Railroads in America have more wyes than railroads elsewhere, and American locomotives and cars are much more likely to be directional than those elsewhere. This is due to the fact that in most places in the United States, the railroad came first, or at least early, and therefore builders had much more freedom to lay down tracks where they wanted. In Europe, extensive use was made historically of bi-directional tank locomotives and push-pull trains, and more recently most diesel locomotives and electric locomotives ordered in Europe have been fully bi-directional.