A flying junction or flyover is a railway junction at which one or more diverging or converging tracks in a multiple-track route cross other tracks on the route by bridge to avoid conflict with other train movements. A more technical term is "grade-separated junction". A burrowing junction or dive-under occurs where the diverging line passes below the main line.
Flying junction just east of Columbia station in New Westminster in Canada
The Uptown Hudson Tubes in Jersey City, New Jersey were built c. 1910.
Flying junction on the Tremont Street subway approaching the Pleasant Street incline in Boston
Zoo Junction in Philadelphia in 1977
In civil engineering, grade separation is a method of aligning a junction of two or more surface transport axes at different heights (grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges, tunnels, or a combination of both can be built at a junction to achieve the needed grade separation.
An example of the potential complexity of grade separation, seen in the Jane Byrne Interchange in Chicago
Seven various overpasses for grade separation in Spain near Barcelona
Rail-rail grade separation in Xiaoshan, China
The concept of grade separation includes all transport modes, such as a simple pedestrian bridge over rail tracks.
