Page:Popular Science Monthly Volume 74.djvu/216

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

which these facts may go. A steam road in the middle west was paralleled by an electric line; the latter took away over fifty per cent, of the steam traffic and increased the total traffic fifteen times the original amount within seven years. If we look for the reasons for such advantages in absorbing and promoting traffic we realize that electric travel is faster, more frequent and more comfortable. It provides freedom from smoke, better ventilation, easy regulation of light and heat and in fact travel in many instances is actually a pleasure.

While the public is appealed to as cited above, the operating and transportation departments of the railroad are equally appealed to by the methods afforded under electric operation for handling the resulting increase in business. Present day steam service may be divided into three broad classes: (1) Suburban and terminal; (2) long haul passenger and express and (3) freight traffic. The advantages of electricity as motive power in all three of these classes of service are found in the possibilities of obtaining more mileage and hauling capacity from the equipment, the operation of more trains on the line at one time and better operating conditions and consequent reliability.

Figures have already been given showing the increased train capacity of the several electric installations in and about New York city and many other similar instances might be cited. These results are largely due to the fact that an electric locomotive or train operates at a higher schedule speed than is possible under steam. The elapsed time of a suburban train between stops depends principally on the rapidity with which it attains its maximum speed. The rate at which it reaches this speed, that is, the acceleration, depends on the value of the pull exerted by the motive power. During the period of starting the greatest draw-bar pull is required, since during this time the inertia of the mass of the train must be overcome. After reaching a maximum speed the only forces to be overcome are those of frictional resistance of the track and the air. In a steam locomotive the greatest draw-bar pull being required at starting, steam is admitted to the cylinders throughout the full length of the stroke. The demand on the boiler per revolution is, therefore, greatest at this time. No loco-, motive can, on the average, exert a greater pull than 25 per cent, of the weight carried by its own driving wheels, for beyond this figure the wheels will slip on the track. The boiler capacity, therefore, is designed to give no more steam than that demanded by this value of the pull at starting. A steam train, therefore, does not utilize the weight of its own cars as a means of increasing its grip on the rails. In a multiple unit electric train, motors are placed on each car, thus utilizing the weight of the entire train for frictional adhesion to the rail. By electric control of the motor switches all the motors may be operated simultaneously by one man at the head of the train. By this