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.