A double-track railway usually involves running one track in each direction, compared to a single-track railway where trains in both directions share the same track.

In the earliest days of railways in the United Kingdom, most lines were built as double-track because of the difficulty of co-ordinating operations before the invention of the telegraph. The lines also tended to be busy enough to be beyond the capacity of a single track. In the early days the Board of Trade did not consider any single-track railway line to be complete.

In the earliest days of railways in the United States most lines were built as single-track for reasons of cost, and very inefficient timetable working systems were used to prevent head-on collisions on single lines. This improved with the development of the telegraph and the train order system.

In any given country, rail traffic generally runs to one side of a double-track line, not always the same side as road traffic. Thus in Belgium, China, France (apart from the former German Alsace and Lorraine), Sweden, Switzerland and Italy for example, the railways use left-hand running, while the roads use right-hand running. In countries such as Indonesia, it is the reverse (right-hand running for railways and left-hand running for roads).