A subway is an underground railroad system designed for efficient urban and suburban passenger transport. The tunnels usually follow the lines of streets, for ease of constructions by the cut-and-cover method in which an arched tunnel is built in an open trench, covered with earth and the street restored. Outlying parts of the system usually emerge to the surface.

The first subway was built in London (1860–1863) by the cut-and-cover method; it used steam trains and was a success despite fumes. A three-mile section of London subway was built (1886–1890) using a shield developed by J. H. Greathead: this is a large cylindrical steel tube forced forward through the clay by hydraulic jacks; the clay is removed and the tunnel walls built. Deep tunnels are thus possible, and there is no surface disturbance. This London "tube" was the first to use electrically-powered trains, which soon replaced steam trains everywhere. Elevators were provided for the deep stations, later mostly replaced by escalators. Many cities throughout the world followed London's lead, notably Paris (the Métro, begun 1898) and New York (begun 1900). With increasing road traffic in the second half of the 20th century, the value of subways was apparent, and many cities extended, improved, and automated their systems; some introduced quieter rubber-tired trains running on concrete guideways.