Page:Popular Science Monthly Volume 74.djvu/217

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213
ELECTRIC OPERATIONS OF STEAM RAILWAYS

system the draw-bar pull per ton of train may be increased from 2.5 to 4.5 times that for steam and the rate of acceleration is only limited by the comfort of the passengers. As a direct result of this the schedule speed is increased;, the headway between trains is reduced and more trains may be operated on the line. By the use of electric locomotives in terminals for switching service great economies are effected. Since the electric locomotive operates in either direction and takes its entire power supply from the trolley or third rail, much useless mileage of locomotives in going to and from the turn-table, the water-tank and the coal-chute is avoided. The New York Central has already reported as a result of tests a net saving of 21 per cent, on the cost of switching service and 16 per cent, in the ton mileage of switching locomotives.

For long-haul passenger and express service rapid acceleration is not so important, but the maximum speed becomes the determining factor in a fast schedule. For any type of motive power the draw-bar pull is greatest at starting and falls to lower and lower values as maximum speed is approached. Consequently, for this class of service, large initial effort is not so important as large effort at high speed. In this respect the electric motor has a great advantage over the steam engine. Since the boiler of the steam locomotive is proportioned to the maximum demand which it can generate at starting, corresponding to the grip which it has on the rails, at higher speeds the steam must be cut off from the cylinders at a less and less fraction of full stroke, for otherwise the boiler can not supply steam fast enough and still maintain its pressure; thus the total tractive effort, which depends on the proportion of a revolution during which steam is admitted to the cylinders, is reduced as the speed increases. While the tractive effort of the electric motor also decreases somewhat with the speed it does not do so nearly as rapidly as that of the steam locomotive. As a consequence, a given weight of train can be handled faster by electricity than by steam or a heavier train may be hauled at a given maximum speed. Again, the safe limits of speed are much higher in electric operation. The rotative effort is uniform in a motor, while that of a locomotive is intermittent and accomplished through the medium of heavy reciprocating parts. The moving mass of these parts as the speed increases tends to lift the locomotive from the track and pounds the rails with a blow which in many instances has been sufficient to cause derailments. The limiting speed of steam trains is about 80 or 90 miles per hour, while speeds of 130 miles per hour have been reached in tests on electric trains.

The advantages of electricity for freight traffic are most apparent on long single track lines with heavy grades and mixed traffic. The length of the freight train of to-day is limited by the draw-bar pull of the locomotive which is in turn dependent on the locomotive weight.