Underground railway systems used to transport large numbers of people within urban and suburban areas typically by means of trains comprised of a number of cars operated on the multiple-unit system. Subways are usually built under city streets for ease of construction, but they sometimes must pass under rivers. Outlying sections of the system usually emerge aboveground, blending into conventional railways or elevated transit lines.
Source: AAT
http://vocab.getty.edu/page/aat/300008577
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