The remarkable bus straddles two lanes of traffic, allowing cars to drive underneath while it carries up to 1,200 passengers.
It's environmentally sound too because it runs on electricity, using a state-of-the-art charging system. Called relay charging, the roof of the bus conducts electricity and contacts special charging posts as it moves along.
Engadget links to a video in Chinese explaining the concept, which is pretty clear even if you don't know Chinese. A trial line is planned in Beijing, so we won't have to debate it in theory for much longer.
But this is interesting:
It's cost-effective because there are two ways it could operate: first off, special tracks could be laid into each side of the road, like a tram.
Or secondly, simple coloured lines could be painted onto the road for it to follow automatically on conventional tyres. There'll be a driver on the bus at all times, though.
I'm not sure how that makes it cost-effective, but it does have the effect of reducing the bus-rail distinction an almost academic quibble.
Either way, this is going to be a large structure resting on narrow wheels. It could be on rubber tires but linked to an optical-guidance system (sensors on the vehicle responding to a painted line on the pavement) and the effect would be the same as if it were on rails: a controlled path with little or no lateral motion.
So is it a train or a bus? Who cares?