Dubbed the AutoTram, the vehicle is powered by a fuel-cell motor and a massive flywheel that keeps rotating at stops, giving the AutoTram extra acceleration when it gets moving again.

The prototype developed by the Fraunhofer Institute for Transport and Infrastructure Systems (IVI) in Dresden is 18 metres long, but three can be coupled together into a road train 56 metres long that can carry the full complement of 300 people.

Although it runs on regular rubber tyres, engineers say the vehicle has the advantages of a tram without the cost of having to lay tracks and electric wires: high capacity and no risk of scuffing heritage buildings in historic city centres.

"It has an optical tracking system," explained chief engineer Matthias Klingner. The sensors look for lines painted on the road, and help the driver stay on track. All three axles of the prototype can be steered, making the vehicle more manoeuvrable.

The power system is an 80-kilowatt fuel cell supplied by Ballard Power Systems, backed up by a compact, Dutch-made CCM Nuenen flywheel that can kick in with as much as 325 kilowatts when the AutoTram needs a power boost.

The engineers explained that the main engine is fairly puny for a vehicle this size, but that municipal buses only operate at full power in short spurts. That makes it more sensible to generate and store up energy while the bus is at stops or red lights.

Hybrid cars like the Toyota Prius save this energy to an electric battery, but the AutoTram engineers believe it makes more sense to save it as rotational energy in the flywheel, since it can be fed straight back to the wheels as needed.

More fuel can be saved by generating power off the brakes of the AutoTram. The institute claims the AutoTram runs on 25 per cent less fuel than comparable municipal buses, and keeps city air clean: the exhaust gas is almost entirely composed of water vapour.

Copyright 2005 dpa Deutsche Presse-Agentur GmbH