Electricity in Locomotion / An Account of Its Mechanism, Its Achievements, and Its Prospects

by Adam Gowens Whyte

The crowning advantage of electric traction lies, however, in the more rapid acceleration which it affords. We have already seen how important this item is on tramways. It is still more important on suburban railways, where a high average speed, in spite of frequent stops, is a vital matter.

On the District Railway the rate of acceleration in the old steam days was about 6 inches per second per second. It was, in fact, so low that the trains could not reach a fair speed before the brakes had to be applied to bring the train to a stop at the next station. With electric traction the rate of acceleration has risen to about 18 inches per second per second. On the Liverpool Overhead Railway a rate of 36 inches per second per second was reached in certain tests. Heavy starting currents are, of course, necessary to bring a train from rest to full speed at such a rapid rate, but it is quite possible for the electrical engineer, without being unduly extravagant in current, to accelerate a train more quickly than the passengers would find comfortable.

The practical result of rapid acceleration (combined with rapid braking) is not only to give a higher average speed but also to enable a more frequent service to be run. Owing to the block system on railways it is impossible for trains to follow each other closely in the manner of tramcars; and it is therefore of cardinal importance that no train should occupy a block for one second more than is necessary. Rapid acceleration becomes all the more important in this respect because of the difficulty of setting down and picking up passengers quickly. This difficulty is overcome in part by using saloon carriages with middle and end doors, in place of compartment carriages. At first the District Railway tried to help matters by operating these doors pneumatically, but the mechanism became unpopular after a number of late-comers had been pinched by closing doors. The management has reverted to hand operation; and it has probably achieved more by educating the public to move quickly than it would have gained with its too-perfect mechanical system.

London travellers have become so accustomed to entering and leaving trains quickly that it is possible for an observer to distinguish strangers by their slower movements on an underground railway. Thus the passenger, as well as the service, has been 'speeded-up.' The more frequent service of trains with a higher average speed would not have been possible, however, without an improvement upon the old methods of signalling. There is no need to dwell upon the weakness of the human element in railway signalling; and it will be clear even to the layman that the strain of handling traffic with a headway of one minute and a half, or less, would be more than men could stand. Automatic signalling had therefore to be adopted to obviate the risk of disaster.

Each train, as it leaves a block or section, 'clears' the signals for that block; and when any train attempts to enter a block against signals, the current is automatically switched off and the brakes applied. The system is so perfect that, in spite of the enormous traffic worked under it, there has been no failure and no accident. It is, of course, costly to install; and its cost can be justified (financially) only when the traffic is very heavy—that is to say, when the conditions make it almost a necessity.

The supply of electric power to electric railways is organised on practically the same lines as in the case of tramways. That is to say, current is generated at a central station, transmitted at high pressure to various sub-stations, and supplied from there at working pressure through 'feeders' to each section of the system. In the case of the 'Underground' system, most of the power is taken from a single huge electric station at Chelsea. Current from that station drives trains as far west as Wimbledon, Hounslow, and Ealing, as far north as Highgate and Golder's Green, and as far east as Barking.

This is a magnificent example of the concentration which gives economy. If each of the underground railways forming the system had erected its own generating station, the total initial outlay, on land, buildings, and machinery, would have been greater, and the cost of current would have been higher, owing to the smaller output and the more irregular demand which a single railway affords. The ideal electric power station is one which is constructed with the largest generating units and produces current at its maximum capacity throughout the twenty-four hours of each day. The Chelsea power station is nearer the ideal than a smaller one supplying a short railway could be. And a station of the latter class is, it may be noted, nearer the ideal than the arrangements on a steam railway, where the sources of power are scattered in hundreds of locomotives.

The concentration of power is therefore one of the many factors which have enabled electric railways to give a vastly improved service at lower fares.

With two exceptions—to be considered in the next chapter—the electric railways of Great Britain are constructed on the 'third-rail' system. They are thus a reversion to—or, rather, a survival of—the original type adopted by Siemens in 1879. The 'third-rail' is carried on insulators a few inches outside the track rail; and the motor cars are provided with a 'brush' or 'shoe' which slides along it and collects the current. In the centre of the track there is generally a second insulated rail to carry the return current, as it is more convenient, under railway conditions, to have a conductor independent of the track rails than to follow the tramway plan of using the rails 'bonded' together. In stations and at crossings the third or 'live' rail is protected by a wooden board in order to reduce the risk of shock to anyone falling on the line or walking upon it. The board is placed high enough over the rail to allow the shoe to pass freely.

As regards the motor equipment on the cars, tramway models have been followed very closely. The 'series-parallel' system of control is again adopted in order to get the high starting torque which gives rapid acceleration with moderate current consumption. The course of the current is again from the live rail, through the controller, through the motors, and thence to the return rail. The controller itself is more or less on the tramway principle; and the main modification in it is the arrangement which enables all the motors on a multiple-unit train to be operated by a single controller. This is done by connecting the controllers electrically and using electric power so that they all work in unison. Some companies use, for this purpose, compressed air controlled by electricity instead of electric power alone, but in both cases the principle is essentially the same.