The Great Western Railway branch line from Yelverton to Princetown opened in 1883, amongst other things to serve Dartmoor prison and the local granite industry. Princetown station, said to be the highest railway station in England at 1,373 feet above sea level, was reached after almost continuous gradient from Yelverton which itself was 500 feet about sea level. The sharply-curved line between Yelverton towards Princetown had ruling gradients between 1 in 40 and 1 in 50 for much of its length: the 20 mph speed restriction in this direction was felt to be unnecessary!
Two of the 44XX class 2-6-2 locomotives built in 1905 normally worked the line, usually with the chimney end of the locomotive towards Princetown to keep the firebox crown covered on the gradients. These locomotives had 4ft 1½ inch diameter driving wheels for working branch lines in hilly areas and the class was renowned for rapid acceleration. Number 4402 (pictured) is believed to have spent almost its entire life working the Princetown branch. 4402 was experimentally fitted in 1931 with a Westinghouse compressed air brake to boost braking power. Another refinement added at the same time was a Westinghouse air-operated atomiser system which blew a fine mist of gas oil from oil containers fitted near the water tanks onto the lower part of the driving wheel flanges. The aim was to reduce wear on wheel flanges and rails on this heavily curved line. But it was found that the oil spread to the wheel tyres leading to excessive slipping so the equipment was removed. Then locomotive 4407 was fitted with a gravity-fed flange oiling system but this failed as the oil was blown by Dartmoor’s strong winds onto the tyres and more slipping was inevitable. A later system was tried in which coolant (similar to diluted machine cutting oil) stored in reservoirs by the side of the smokebox was sprayed under steam pressure onto the rails in front of the leading wheels.
In an attempt to attract tourist traffic, new halts on the line were opened at Burrator & Sheepstor in 1924, King Tor in 1928 and Ingra Tor in 1936. Ingra Tor Halt was noted for its public notice warning of snakes! Hardy souls could also benefit from a £4 per week eight-berth camping coach stationed at Princetown during the 1930s. This gave unrivalled views of chained and escorted prisoners arriving for their stay on Dartmoor.