Rapid transit
Trains

Most rapid transit trains are electric multiple units with lengths from three to over ten cars. Crew sizes have decreased throughout history, with some modern systems now running completely unstaffed trains. Other trains continue to have drivers, even if their only role in normal operation is to open and close the doors of the trains at stations. Power is commonly delivered by a third rail or by overhead wires. The whole London Underground network uses fourth rail and others use the linear motor for propulsion.

Some urban rail lines are built to a loading gauge as large as that of main-line railways; others are built to a smaller one and have tunnels that restrict the size and sometimes the shape of the train compartments. One example is most of the London Underground, which has acquired the informal term "tube train" due to the cylindrical shape of the trains used on the deep tube lines.

Historically, rapid transit trains used ceiling fans and openable windows to provide fresh air and piston-effect wind cooling to riders. From the 1950s to the 1990s (and in most of Europe until the 2000s), many rapid transit trains from that era were also fitted with forced-air ventilation systems in carriage ceiling units for passenger comfort. Early rapid transit rolling stock fitted with air conditioning, such as the Hudson and Manhattan Railroad K-series cars from 1958, the New York City Subway R38 and R42 cars from the late-1960s, and the Nagoya Municipal Subway 3000 series, Osaka Municipal Subway 10 series and MTR M-Train EMUs from the 1970s, were generally only made possible largely due to the relatively generous loading gauges of these systems and also adequate open-air sections to dissipate hot air from these air conditioning units. Especially in some rapid transit systems such as the Montreal Metro (opened 1966) and Sapporo Municipal Subway (opened 1971), their entirely enclosed nature due to their use of rubber-tyred technology to cope with heavy snowfall experienced by both cities in winter precludes any air-conditioning retrofits of rolling stock due to the risk of heating the tunnels to temperatures that would be too hot for passengers and for train operations.

In many cities, metro networks consist of lines operating different sizes and types of vehicles. Although these sub-networks may not often be connected by track, in cases when it is necessary, rolling stock with a smaller loading gauge from one sub network may be transported along other lines that use larger trains. On some networks such operations are part of normal services.
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