The class 27 was a passenger-only development of the class 26. Like its predecessor, it was built by Birmingham Railway Carriage and Wagon Company and was called BRCW Type 2. It had a top speed of 90 mph (145 km/h) and the output of the six-cylinder Sulzer engine had been increased to 1,250 hp. In 1961 and 1962, a total of 69 were completed. By 1969, the whole class had been concentrated in Scotland.

Only a short time later, 24 were converted for push-pull operation between Edinburgh and Glasgow. These were designated class 27/1 and used with passenger carriages with wiring to control the locomotive at the rear of the train from the leading one. Later half of these locomotives were rebuilt to class 27/2 by adding a 160 hp Deutz diesel engine with generator as a replacement for the earlier steam boiler. Now each push-pull train had one class 27/1 and one class 27/2 locomotive. The last one was withdrawn in August 1987.