What is a subway?
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.
