What is a Trolleybus?

	Definition: A trolleybus is an electrically-powered bus which runs on tyres and draws its power from two overhead conductors (one live, one return). See the picture below where the two overhead wires can be seen.
	Two trolleybuses overtake stationary motorbuses.
	Details: A trolleybus is steered by a driver in the same way as an ordinary bus, but the controls (accelerator and brake – there is no clutch or gearbox) govern the way the power is supplied to an electric motor which drives the wheels directly. Electric current is picked up from two parallel wires in the air (generally two feet apart). Swivelling booms - or trolley-poles - on the roof make contact with the wires through carbon sliders fitted into the swivelling trolley-heads. Unlike a tram, a trolleybus can operate at a short distance from the wires (dictated by the length of the booms), and so can negotiate around obstacles - see the picture above. Straying too far from the wires causes a dewirement where the trolley-heads part company with the wires, and they have to be replaced by the driver. This used to be by the use of a long pole, but generally modern systems use retriever ropes attached to sprung drums on the back of the vehicle. These double as safety devices, stopping the booms springing up too far in the event of a dewirement which can cause damage to the overhead. In addition lightweight booms and modern overhead construction methods make modern trolleybuses much less likely to 'dewire' in the first place. The electricity supplied to the motor in the vehicle is usually dc, the power supplied through the overhead being generally dc also (generally at 550-600 volts, although some systems use 750 volts, and some even went as high as 1000 volts). The vehicle control system used to consist of a number of contactors, or switches, which governed the way the power was applied to the motor. Modern solid-state methods, however, now mean that the power can be gradually applied without the ‘steps’ created by contactors, supplying a simpler ac induction motor via an inverter system. Some trolleybuses (and most modern ones) have some form of alternative power source - a small diesel engine or a bank of batteries. These enable them to operate ‘off-wire’ for a short distance, enabling them to get round larger problems (a closed street, or a power failure), or return to depot if they suffer main electrical failure. Some trolleybuses are fitted with a full-sized diesel engine, either mechanically coupled to the wheels in addition to the electric motor, or using a directly-connected generator to supply electricity to the motor. These are commonly known as duo-buses, and can operate as effectively on diesel power or from the overhead in electric mode. These vehicles are used where there is a high-volume corridor where the use of overhead wires is efficient, then operating on diesel power in, say, the suburbs where the intensity of service is not sufficient to support the erection of the expensive overhead. Conversely, there may be an area in a city centre where overhead is not permitted for aesthetic reasons. Duo-buses tend to be complex and expensive, however, as they need to have the equipment associated with both sorts of drive. They are also less efficient as they have to carry the weight of both sets of equipment, while only using one at once.
	Origins: The word ‘trolleybus’ originates from one of the forms of power pick-up that the early examples used. This was in the form of a ‘trolley’ or 'troller' which ran along the top of the overhead wires, the four wheels picking up the current. Both horizontal and vertical examples were experimented with. It was not long, however, before sprung booms attached to swivelling trolley-bases on the roof of the vehicle, took over the role of contacting the wires (see picture above). The contact device started off as a wheel (echoing the trolley collection method, but now on the underside of the wires), but later changed to a sliding swivelling head with a carbon insert which could be replaced as it wore out, rather than wearing out the more costly overhead wire. Many early systems called trolleybuses 'tracklesses' or 'the trackless' which was coined because many trolleybus systems took over from tramways and of course the trolleybuses didn't need tracks, hence the name.
	Tram: A tram is an electrically-powered vehicle which runs on rails and draws its power from one overhead conductor, and returns it through the rails. The driver does not need to steer, simply controlling the speed of the tram, and the direction it takes at junctions.
	Hybrids: There are a number of hybrid systems which have elements of both trolleybus and tram. For example, there are a number of guided trolleybus systems now possible, like the optical guidance system where the vehicle is steered by a detector following some sort of line along the road. This would still be regarded as a trolleybus. Note that guidance for a rubber-tyred vehicle is always optional - a trolleybus does not have to be guided. Some guidance systems are more mechanically linked; eg. the guided busway concept, where the wheels are steered by control arms running along the edge of a trough along which the vehicle runs. This has to be supplemented by driver control in places, however, as the vehicle cannot run everywhere in this mode – pointwork is impractical. An overhead-powered electric vehicle in this mode would still be regarded as a trolleybus. Some guidance systems have used one guide rail along the middle of the road, but the vehicle wheels are still tyres, and this would still be regarded as a trolleybus. However, if that guide rail is used as the return for the power, and the overhead is reduced from two wires to one, then I will leave you to decide whether this is a trolleybus or a tram!