A single-track railway is a railway where trains traveling 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 and maintaining a second track. Advantages and disadvantages 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 safely. By contrast, a double track with signal boxes four minutes apart can allow up to 15 trains per hour in each direction safely, 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 propagation of 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. Single-track operations Passing loops 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 or L202 railway in Croatia) 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. Safety operations 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