A single trackrailway 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. Telegraphy (from the Greek words tele = far away and grapho = write) is the long distance transmission of written messages without physical transport of letters, originally over wire. ...
Some early wagonways, such as the Gloucester and Cheltenham Railway, were primarily single track with crossing loops at frequent intervals. The crossing loops were arrangement 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. Wagonways are the horses, equipment, and tracks used for hauling wagons which preceded steam powered railways. ... A crossing loop is a place on a single line railway where trains in opposing directions can cross each other. ... A chord of a curve is a geometric line segment whose endpoints both lie on the curve. ...
When a single track railway is converted to a double track railway, in some countries this is called duplication. The converse operation, converting a double track railway to single track, is known as singling.
If singletrack construction is used, then there need to be periodic passing sidings where a car traveling in one direction can wait while one going in the opposite direction passes (see photos).
Singletrack construction is considerably cheaper to build but has lower capacity as the number of cars operating in each direction is limited by the spacing of passing sidings.
Double track construction costs are higher than for singletrack, but the capacity is much higher as streetcars traveling in opposite directions may pass at any point.
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