ELECTRIC OPERATIONS OF STEAM RAILWAYS 213 



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 maxi- 

 mum 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 con- 

 sequence, 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. 



