A rapid transit system that primarily or traditionally runs below the surface may be called a subway, tube, or underground. Unlike buses or trams, rapid transit systems are railways, usually electric, that operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles.

Technically and legally, a subway is indeed a type of railroad, but it is specifically classified as "rapid transit" or "heavy rail." In a broad engineering sense, any system involving wheeled vehicles running on fixed rails is a railway. However, subways differ from traditional "mainline" railroads (like Amtrak or Union Pacific) in that they are grade-separated, meaning the tracks are completely isolated from street traffic and pedestrians, usually via tunnels or elevated structures. Legally, in the United States, most subway systems are not governed by the Federal Railroad Administration (FRA) because they do not share tracks with the general national railroad system; instead, they fall under the jurisdiction of the Federal Transit Administration (FTA) and state-level safety oversight. There are rare exceptions, such as the PATH train in New York/New Jersey or the Staten Island Railway, which are considered "railroads" by the FRA due to their historical connections to the national rail network. In most of Europe and Asia, the term "metro" is used to distinguish these urban high-capacity electric railways from long-distance passenger or freight trains.