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There is no single term in English that all speakers would use for all rapid transit or metro systems.This fact reflects variations not only in national and regional usage，but in what characteristics are considered essential.

One definition of a metro system is as follows：an urban electric mass transit railway system totally independent from other traffic with high service frequency.

But those who prefer the American term“subway”or the British“underground”would additionally specify that the tracks and stations must be located below street level so that pedestrians and road users see the street exactly as it would be without the subway；or at least that this must be true for the most important，central parts of the system.A rapid transit system that is generally above street level may be called an“elevated”system（often shortened to el or，in Chicago，“L”）.In some cities the word“subway”applies to the entire system，in others only to those parts that actually are underground and analogously for“el”.

The first real underground line was the Metropolitan Railway in London，which opened in 1863，using the era's most advanced propulsive technology：steam locomotives，especially designed to condense their exhaust steam when in the tunnels.It was an immediate success and many extensions followed.The Metropolitan finally became an important part of the London Underground system.Steam working underground lasted until 1905.

The first elevated railway in the world was the Ninth Avenue Elevated in New York City，opened in 1868 as a cable car and later changed to steam and then electric operation.Elevated railways were seen as a cheaper alternative to subways，but were often seen as dirty，ugly，and dangerous.

The oldest subway in the Southern Hemisphere opened in 1913 in Buenos Aires，Argentina，which is also the oldest one in Latin America and the whole Spanish-speaking world.

Asia's oldest subway line is Tokyo's Ginza Line，opened in 1927.Now there are 12 subway lines running on about 150 miles of track.Other major Japanese cities also have subway systems，including Yokohama，Osaka，Nagoya，Sapporo，Kobe，Kyoto，Fukuoka，and Sendai.

In 1979，Hong Kong's subway line，now called MTR，began operations.It presently has seven lines，including four that run underneath Victoria Harbor.