The most common type of track used around the world is ‘flat bottom’ steel rails supported on sleepers made of timber or pre-stressed concrete. Typically, tracks are laid on a bed of stone track ballast which is supported by prepared earthworks, known as the track formation.

Ballast consists of crushed stone and is used to uniformly distribute loads from the sleepers to the formation. Ballast also helps with stabilisation, drainage and prevents the sleepers from moving. It is typically laid to a depth of between 225-300 mm, filled to the top of the sleepers and beyond the ends of the sleeper by around 250-500 mm.

The track formation consists of a subgrade and blanket – a layer of sand or stone dust in impervious plastic. The blanket restricts the upward movement of wet clay or silt. Additional waterproofing is provided if deemed necessary. The subgrade layer is slightly sloped at the sides to help water drainage. Water is channelled away by ditch or other type of drain.

In order to prevent soil and water from 'spoiling' the ballast, some railways can use asphalt pavement below the ballast, which helps to stabilise it. Where track is to be laid over permafrost, measures such as transverse pipes of cold air can be laid through the subgrade to prevent melting.

A single-track railway is a railway where trains travelling in both directions share the same track. Single track is usually found on lesser-used rail lines, often branch lines, where the level of traffic is not high enough to justify the cost of constructing a second track. Single track is significantly cheaper to build and maintain, but has operational and safety disadvantages.

For example, a single-track line that takes 15 minutes to travel through would have capacity for only two trains per hour in each direction. By contrast, a double track with signal boxes four minutes apart can allow up to 15 trains per hour in each direction, provided all the trains travel at the same speed. This hindrance on the capacity of a single track may be partly overcome by making the track one-way on alternate days, if the single track is not used for public passenger transit. Long freight trains are a problem if the passing stretches are not long enough. Other disadvantages include the delays, since one delayed train on a single track will also delay any train waiting for it to pass. Also, a single track does not have a "reserve" track that can allow a reduced capacity service to continue if one track is closed.

If a single-track line is designed to be used by more than one train at a time, it must have passing loops (also called passing sidings or crossing loops) at intervals along the line to allow trains running in different directions to pass each other. These consist of short stretches of double track, usually long enough to hold one train. The capacity of a single-track line is determined by the number of passing loops. Passing loops may also be used to allow trains heading in the same direction at different speeds to overtake. In some circumstances on some isolated branch lines with a simple shuttle service (such as the Abbey Line in Great Britain) a single-track line may work under the "one train working" principle without passing loops, where only one train is allowed on the line at a time. On single-track lines with passing loops, measures must be taken to ensure that only one train in one direction can use a stretch of single track at a time, as head-on collisions are a particular risk. Some form of signalling system is required.