Via our own DeputyDog's excellent site.

from the GV poster:

The Railplane was a form of monorail with a carriage held on an overhead track, which itself was suspended on a framework of steel girders, by front and rear grooved wheels. The carriage was driven by fore and aft aircraft propellers, powered by either a live electric rail or petrol engines, and braked by a system acting on the overhead wheels and by rotating the rear propeller in a different direction from the front one. A rigid rail under the carriage prevented swaying.