Single track (rail)

A single track railway is one where traffic in both directions shares the same track. In the early days of railways, especially before the telegraph, operation of significant numbers of trains on a single track railway was fraught with difficulties, including delays and accidents, particularly head-on collisions.

Some early wagonways, such as the Gloucester and Cheltenham Railway, were primarily single track with crossing loops at frequent intervals. The crossing loops were arranged to be in line of sight of one another, so that drivers in one direction could see if vehicles in the opposing direction were already in the single line section. The single line sections needed to be straight, so the profile of the line tended to be a series of chords rather than a smooth arc.

When a single track railway is converted to a double track railway, in some countries this is called duplication or doubling. The converse operation, converting a double track railway to single track, is known as singling.

Though single track is significantly cheaper to build, it has a number of operational disadvantages. Single track typically has only one seventh of the capacity of a double track, rarely allowing more than about three trains an hour per direction, depending on the passing track frequency, while a double track typically can allow between 20 and 30 trains per hour. Also, there can be problems with long freight train if there are not enough long passing stretches, reducing the capacity of the track even more. Other disadvantages include the spread of delays, since if one train on a single track is delayed, any train waiting for it to pass also will be delayed, and will continue on to delay more trains. Also, single track does not have a "reserve" track that can allow a reduced capacity service to continue if one track is closed, but not the other (single-line working).