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
Famous quotes containing the words motive power, train, size, motive and/or power:
“He is the best sailor who can steer within the fewest points of the wind, and extract a motive power out of the greatest obstacles. Most begin to veer and tack as soon as the wind changes from aft, and as within the tropics it does not blow from all points of the compass, there are some harbors which they can never reach.”
—Henry David Thoreau (1817–1862)
“An immoderate fondness for dress, for pleasure, and for sway, are the passions of savages; the passions that occupy those uncivilized beings who have not yet extended the dominion of the mind, or even learned to think with the energy necessary to concatenate that abstract train of thought which produces principles.... that women from their education and the present state of civilized life, are in the same condition, cannot ... be controverted.”
—Mary Wollstonecraft (1759–1797)
“It is very considerably smaller than Australia and British Somaliland put together. As things stand at present there is nothing much the Texans can do about this, and ... they are inclined to shy away from the subject in ordinary conversation, muttering defensively about the size of oranges.”
—Alex Atkinson, British humor writer. repr. In Present Laughter, ed. Alan Coren (1982)
“If, in looking at the lives of princes, courtiers, men of rank and fashion, we must perforce depict them as idle, profligate, and criminal, we must make allowances for the rich men’s failings, and recollect that we, too, were very likely indolent and voluptuous, had we no motive for work, a mortal’s natural taste for pleasure, and the daily temptation of a large income. What could a great peer, with a great castle and park, and a great fortune, do but be splendid and idle?”
—William Makepeace Thackeray (1811–1863)
“Success four flights Thursday morning all against twenty one mile wind started from Level with engine power alone speed through air thirty one miles longest 57 second inform Press home Christmas.”
—Orville Wright (1871–1948)