Regenerative braking is a way of slowing an electrically powered vehicle by using the motors as brakes. Instead of the surplus energy of the vehicle being wasted as unwanted heat, the motors act as generators and return some of it to the overhead wire as electricity.

This means that a tram coming down a hill can help power another one going uphill. Energy savings of 23% were recorded by this method in pre-war Manchester and the hilly nature of Bath points to possibly greater savings.

When tramway electrical systems were connected directly to their own power stations, the regenerated electricity, if not actually being used by another tram at the time, would be returned to the generating plant where it increased the speed of the massive flywheels fitted for energy storage purposes. This automatically cut off steam from the driving engines, saving energy. When another tram had used up the stored energy and flywheel speed returned to normal, the steam valves automatically re-opened, maintaining the correct generator speed. Click here for full-sized picture[76kb]

It is an unfortunate side-effect of modern tramway electrical supply equipment, which is in many other ways superior to its predecessors, that it cannot generally return surplus power to the supply mains. If another tram is not in the area and "receptive", the energy must be wasted as heat in a bank of resistors on the tram which is braking.
(Equipment does exist which can return power, but it is very expensive and a sudden surge of power on the local electricity supply would nowadays be most unwelcome.)