A subway is a train system which is located entirely underground. The first subway was the London Underground, which opened in 1863, and many cities followed suit when they realized how useful a subway could be. Since subways are on a different grade than general traffic, they can operate entirely autonomously, free of traffic and other problems on surface streets. Because a subway uses dedicated tracks, it can also run rapidly and frequently, making it a valuable addition to a public transit network.

Typically, a subway runs on electricity, which is often provided through the so-called “third rail.” The trains of the subway run on two traditional tracks, just as above-ground trains do, and they power themselves through paddles attached to the third rail. This rail represents a substantial danger to people who are foolish enough to jump onto the tracks, since it carries a very high voltage and it is exposed with no insulation. Because of concerns about the third rail, subway tracks are usually submerged below the level of the station platform to discourage jumping, and passengers are reminded to stand clear of the platform edge unless they are boarding.

In some cases, a subway system may integrate above ground or elevated tracks in addition to an underground system. Commonly, the subway remains underground in the downtown section of the city, while elevated tracks branch out into less busy areas. These tracks may also network with other transit systems, such as commuter rail bringing people in from the suburbs, or a city bus system.

Like other forms of public transit, most subways are heavily subsidized by the regions that they run in. These subsidies keep fares relatively inexpensive, encouraging commuters to choose the subway over personal vehicles. When a subway is well run and it networks with other transit, it can play a significant role in reducing general traffic, making a city more pleasant to live, work, and play in.

In many cities, a subway includes multiple track systems which spread out underneath a city for wide coverage. Frequent stations along the tracks ensure that people will be able to quickly reach their destinations. People tend to be pay by the length of their trip, although in some areas passengers use transit passes which bill per trip, or by the amount of time spent on the subway system. The subway company usually makes an effort to make the subway easy to navigate and hospitable for visitors, so that they will not add to traffic congestion during their stay in the city.