The Siemens Combino trams on the streets of Basel, in northern Switzerland, look modern, stylish and elegant. Yet when they go round a corner they look clumsy. The front wheels jolt against the track going into the bend. The rears sway from side to side as they leave it.

The poor ride reflects a conundrum facing tram manufacturers. Over the past 20 years, cities with tram and light rail systems have grown increasingly unhappy with traditional designs which mean vehicle floors are nearly a metre above the track and passengers climb steps to get on.