In course of time, however, it came to be recognised that the function of electricity is not to act like a blue pencil on the debit side of the revenue account. Its essential purpose is to increase the volume of traffic. From the public point of view this is very much more valuable. Passengers are not directly concerned with means of reducing working expenses, but they are closely interested in the improvement of the frequency and speed of the service. The adoption of electricity on suburban lines has really been dictated by the demand for increased facilities. At the 'rush' hours of the morning and evening, when the great tide of workers flows and ebbs, the capacity of the steam lines was taxed to the utmost. And with the growth of population the difficulty of running sufficiently frequent trains became almost insuperable. Apart from these particular necessities, the general features of railway economics point to the supreme advantage of increasing the volume of traffic in every possible way. In a railway, as in a tramway, the preponderating item is the cost of construction and maintenance; and unless a certain minimum of traffic is carried, the most economical working in the world will not secure a profit. The standing charges fall upon the idle hours as well as upon the busy; for every minute that a line of rails stands empty there is a loss of money. Railway progress depends upon reducing the proportion of idle hours; and that can only be done where there is scope for the growth of traffic, and where there is means--such as electric traction--of dealing with that growth on an economical basis. In the succeeding chapter it is explained how electric traction enables a more frequent service to be run with advantage even on systems which were worked to the maximum limit possible under steam conditions. But in the meantime it will be interesting to trace the effect itself on a railway which soon followed the Mersey Railway in making the change from steam to electricity--the Metropolitan District Railway. [Illustration: Fig. 11. An electric train on the Metropolitan District Railway, equipped by the British Thomson Houston Company. The front and rear cars and one intermediate car are equipped with electric motors, all controlled from the 'cab' at the end of the train. The controller handle may be seen close to the nearest window of the first car. The rail immediately in front of the foot of the guard is the conductor rail which conveys the current to the train. The rail between the track rails carries the return current.]