Train Size and Motive Power
Some urban rail lines are built to the full size of main-line railways; others use smaller tunnels, restricting the size and sometimes the shape of the trains (in the London Underground the informal term tube train is commonly used). Some lines use light rail rolling stock, perhaps surface cars simply routed into a tunnel for all or part of their route. In many cities, such as London and Boston's MBTA, lines using different types of vehicles are organised into a single unified system (though often not connected by track).
Although the initial lines of what became the London Underground used steam engines, most metro trains, both now and historically, are electric multiple units, with steel wheels running on two steel rails. Power for the trains, referred to as traction power, is commonly supplied by means of a single live third rail (as in New York) at 600 to 750 volts, but some systems use two live rails (notably London) and thus eliminate the return current from the running rails. Overhead wires, allowing higher voltages, are more likely to be used on metro systems without much length in tunnel, as in Amsterdam; but they also occur on some that are underground, as in Madrid. Boston's Green Line trains derive power from an overhead wire, both while traveling in a tunnel in the central city and at street level in the suburban areas.
Systems generally use DC power rather than AC, even though this requires large rectifiers for the power supply. DC motors were formerly more efficient for railway applications, and once a DC system is in place, converting it to AC is generally considered too large a project to contemplate.
Read more about this topic: Rapid Transit Technology
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