Rapid transit technology

Rapid transit technology describes the use of infrastructure and technology on rapid transit (also called "metro" or "subway" systems). It has changed significantly in the years since the first underground railway lines opened in London in 1863.

Some urban rail lines are built with a loading gauge equivalent to the full size of main-line railways; others use smaller tunnels, restricting the size and sometimes the shape of the trains, as in the London Underground where the informal term tube train is commonly used.

There are lines which use light rail , perhaps trams or streetcars, that are simply routed into a tunnel, viaduct, or other type of grade-separated alignment for all or part of their route (as in Philadelphia, where the tunnels are shared with full-size heavy rail trains). These sections often lead to varying platform heights and may also account for a sizable "gap" between the train and the platform.

In many cities, such as Berlin and Boston, lines using different sizes or types of vehicles are organized into a single unified system. Although these are not often connected by track, in cases when it is necessary, rolling stock among different types of vehicles is compatible for non-revenue transfers or other purposes.

Although the initial lines of what is now the London Underground used steam engines, virtually all metro trains, both now and historically, are electrically powered and built as multiple units. Most use steel wheels running on two steel rails, as in a conventional railway, although some use other methods (described below). Power for the trains, referred to as traction power, is commonly supplied by means of a single live third rail (as in New York). The current powering the trains is generally in the range of 600 to 750 volts, although some systems, such as the ones in London and Milan, use two live rails, one positive and one negative. The use of rails on the ground is mainly due to the limited overhead clearance of the tunnels that often physically prevents the use of overhead wires.

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