A junction, in the context of rail transport, is a place at which two or more rail routes converge or diverge. This implies a physical connection between the tracks of the two routes, provided by points and signalling. Junctions are important for rail systems, their installation into a rail system can expand route capacity, and have a powerful impact upon on-time performance.
Overview
In a simple case where two routes with one or two tracks each meet at a junction, a fairly simple layout of tracks suffices to allow trains to transfer from one route to the other. More complicated junctions are needed to permit trains to travel in either direction after joining the new route, for example by providing a triangular track layout. In this latter case, the three points of the triangle may be given different names, for example using points of the compass as well as the name of the overall place. Rail transport operationsrefer to stations that lie on or near a railway junction as a junction station. In the UK it is customary for the junction to be named after the next station on the branch, e.g. Yeovil Junction is on the mainline railway south of Yeovil, and the next destination on the branch is Yeovil Pen Mill. Frequently, trains are built up and taken apart at such stations so that the same train can split up and go to multiple destinations. For goods trains, marshaling yards serve a similar purpose. The world's first railway junction was Newton Junction near Newton-le-Willows, England where, in 1831, two railways merged.
Measures to improve junction capacity
The capacity of the junctions limits the capacity of a railway network more than the capacity of individual railway lines. This applies more as the network density increases. Measures to improve junctions are often more useful than building new railway lines. The capacity of a railway junction can be increased with improved signaling measures, by building points suitable for higher speeds, or by turning level junctions into flying junctions. With more complicated junctions such construction can rapidly become very expensive, especially if space is restricted by tunnels, bridges or inner-city tracks.
The installation of junctions into a rail system poses many challenges, including increased maintenance costs, and problems in on-time performance. Metro rail systems have a rail network design where the number of junctions is minimized. Passengers, and not trains, move from one train station to another. Metro-style rail systems have become popular because of their efficiency, and high on-time performance. Another solution to removing junctions is to install a flying junction, where tracks are grade separated, and so one track passes over or under another.
