Most of our narrow gauge went just a little bit too early, so although a few lines tried to use petrol locomotives as mainline locos, e.g. Col. Stephen's experiments on the Nogger and the Welsh Highland, but the technology was not quite far enough on, or cheap enough, for it to make it to British NG. One thought that crosses my addled mind is what would have happened had someone had a go with a Simplex based railcar. Motor Rail cut its teeth building petrol trams for the Raj in the Edwardian period. Develop that technology a bit, and make sure they can reliably do 15 - 20 mph and you might have had a winner. Certainly, the Rye and Camber, with its pseudo-Simplex and one coach was only one developmental step off a petrol railcar. One thing that is for sure, the Simplex drive train was very reliable, as evidenced by the geriatric fusiliers still working in places like Leighton Buzzard, and on the Lincs. Coast Light.
To be really pedantic, though, the Clogher Valley Railway operated the first Walker Articulated Railcar from 1931-1941, and was in the UK, just not in Britain.
Peter in Va