CHAPTER XIII
ELECTRIC RAILWAYS FROM THE ENGINEERING POINT OF VIEW
When electric railways were first considered, the natural tendency of engineers was to follow the existing model and merely substitute electric locomotives for steam locomotives. In point of fact, however, the engineering method now adopted is an evolution from the tramway model, not from that of the typical railway.
A certain advantage was, of course, to be gained by replacing steam locomotives by electric ones. The greater 'starting torque' of the electric locomotive enables it to get a train up to full speed more quickly; and the capacity of the electric motor for taking heavy overloads assists the electric train in surmounting heavy gradients. Some advantage was also gained by producing all the power at a central source, instead of having a large number of steam locomotives, which are really power stations on wheels. But the electric locomotive had still to be made heavy enough to get sufficient grip of the rails; it had to haul its own dead weight; and it had to be made powerful enough to tackle a full-sized train on the steepest gradient with its complement of passengers, although the general demand upon it might be considerably less than that maximum.
The electric locomotive, in short, was an advance upon the steam locomotive, but it did not get past the essential drawbacks of the locomotive system. A locomotive is most economical when hauling full trains for long distances at a uniform speed; it is essentially a long-distance machine. The first demand for electrification came, however, from suburban railways, where the stations are close together and where, therefore, the speed is constantly varying from zero up to a maximum and back to zero again. The traffic also fluctuates between extreme limits; and there is obvious waste in having to run heavy locomotives and trains backwards and forwards during the slack hours. There was therefore a demand for some method of propulsion which would enable the length of trains and the consumption of power to be adjusted more closely to the variations in the traffic.
A step in the right direction was taken when the locomotive equipment was placed on a car, thus utilising the weight of the passengers to increase the adhesion on the rails. But the full advantages of electric traction were not realised until what is known as the 'multiple-unit' system was adopted.
The idea underlying this system is quite simple. If, instead of concentrating the motive power on a single locomotive or driving unit, we distribute it among the cars forming a train, we get the multiple-unit system. An electric tramcar and a trailer attached to another tramcar and trailer, with a third tramcar behind, would form a model for a multiple-unit train. By connecting the electrical equipments on the three tramcars—front, middle, and rear—it would be possible to control the train from either end or from the middle.
This is the principle upon which all the electric railways in Great Britain are now worked, with the exception of the City and South London Railway, where locomotives are still used and where the trains are comparatively short and light.
It will be seen that each multiple-unit train is readily divisible. A single motor car may be run, or a car with one or two trailers, or a long train made up of as many motor cars and trailers as the platforms will accommodate. And whether the trains are long or short, the power absorbed is in proportion to the length of the train and the load of passengers. By this simple means power is economised, and the railway engineer is able to reduce the proportion of idle rolling stock.
The adjustment of the length of trains to the fluctuations of the service is made easier by the absence, in the multiple-unit system, of the necessity of shunting at the termini. As a multiple-unit train can be controlled from either end, a more frequent as well as a more flexible service can be run. With steam traction the number of trains which may enter or leave a terminus is limited by the time occupied in shunting and by the necessity of leaving lines of rails free for that operation. With an electric train on the multiple-unit system, no more time is lost than the few seconds necessary for the driver to walk from the front of the train to the rear, which then becomes the 'front.' No lines have to be kept open for shunting locomotives, so that the available accommodation for trains is considerably increased. Some of the London railway companies have spent enormous sums in enlarging their terminal accommodation and have found that it is still inadequate to the demands of the 'rush' traffic. Electric traction therefore offers them an improvement of enormous value without the expenditure of a penny on station alterations.