The Roadrail system was an attempt to combine the advantages of road and rail transport, for use in remote areas where only narrow-gauge rails combined with sharp corners and steep inclines were economically viable. The idea is that one or two bogies underneath the locomotive (or 'tractor' as it was usually called) ran on rails for directional guidance, but propulsion was by means of road wheels contacting prepared ground on either side. The rail wheels had very low rolling resistance on the rails, while the road wheels had much more grip when going up an incline. The system was promoted by Roadrails Ltd of London.
This system seems to have had some success but is today almost unknown. I am grateful to Andy Chapman of the Sentinel Drivers Club for alerting me to this strange business, and providing a copy of a booklet entitled 'The Roadrail System of Traction' from which the images here are derived.
Left: The cover of the Roadrail booklet
The illustration appears to be based on the double-bogie steam tractor illustrated below.
The train appears here to be going up something like a 1 in 3 gradient, which was perhaps a bit optimistic. The usual maximum gradient quoted was 1 in 12, which is still much steeper than a conventional adhesion locomotive could climb.
Left: Double-bogie Roadrail steam tractor
This version of tractor has rail bogies at front and back, with a pair of roadwheels in the middle. There appears to be a vertical boiler at the front of the cab.
There is a remarkable website covering this sort of thing. See Stronach-Dutton road-rail
which has a 27-minute film of roadrail in operation, taken in 1925. It is clearly a seriously wobbly form of transport, because no great care appears to have been taken with the prepared ground on either side of the track, causing the tractor to roll back and forth like a ship in a rough sea. The waggons being pulled also roll badly, presumably because they are wide vehicles carried on narrow-gauge rails (2-foot in general) so any track irregularities are exaggerated.
Page 3 from the Roadrail booklet
The system was used in South Africa using 2-foot-gauge rails.
It was also used in construction work for the British Empire Exhibition at Wembley Park, London, and later transporting visitors around the site.
The system was apparently also used in Scotland, Spain, Morocco, Palestine, Uganda, Australia, Tasmania, and India. Investigations are proceeding.