A high average speed with numerous stops was, however, only one of the improvements which the public derived from electric traction. Tramway passengers expect to find a car not only at a convenient point but within a convenient period of waiting. With electric traction the service became much more frequent than with horse traction. It is quite possible to run a horse tramway service profitably with cars at intervals of fifteen to thirty minutes, if the passengers are patient enough to wait and fill each vehicle. But with electric traction the main item is the cost of the standing equipment--the power house, mains, and overhead lines--and unless that equipment is adequately utilised the revenue will not cover the standing charges. A fifteen-minute service is, generally speaking, the lowest economic limit on an electric tramway. Every tramway manager tries to attract sufficient passengers for a more frequent service; and, as a matter of fact, it was found that where there was sufficient population the provision of a frequent and rapid service encouraged tramway travelling so much that cars had to be run at far shorter intervals than had been customary on horse tramways. The increase of traffic brought with it the demand for larger as well as speedier cars with a shorter 'headway' or interval between one car and another. The capacity of a horse car is limited by the fact that it is not convenient to harness more than two horses to a single vehicle. But with electric cars there is no extraneous limitation to carrying capacity. Large double-decked cars with seats for seventy passengers are now quite common. In America it is a frequent practice to attach 'trailers' to the cars, making a short tramway train. Experiments have recently been arranged on similar lines in London, for the handling of the heavy traffic at rush hours. These instances show that electric tramway capacity is flexible and may be adjusted to the density and the fluctuating character of the demand. Finally, it falls to be noted that the power consumed by a tramcar is, roughly, proportional to the useful work which the car performs. As already mentioned, it costs about as much to work a horse tramway when the cars are empty as when they are full, since the main item is the maintenance of the 'vital principle' of a certain number of horses independently of the traffic. But with electric traction the motors require less power when the cars are running light. And less current for the motors means less current generated at the power station--that is to say, less steam, less oil, less coal, less wear and tear. If more current is demanded, it is because more passengers are being carried and more revenue earned.