Northern Rock was built at the workshops at Ravenglass. Comparative trials had taken place with 4-6-2 Northern Chief from the Romney Hythe and Dymchurch Railway in 1971 which enabled the best features on 15-inch gauge locomotive design to be used. The boiler and enlarged cylinders were based on River Esk, the regulator and springing based on Northern Chief, the outside frames and sliding centre axle were drawn from River Irt.
Overall, a narrow gauge outline was provided which matches the saloon coaches and enabled robust motion parts to minimise maintenance whilst remaining in scale with the loco.
Construction began in 1973 to use a new boiler built earlier by Hunslet in Leeds. Other parts came as raw plates and castings for machining and assembly at Ravenglass. The unfinished loco was first steamed in March 1976. It was commissioned at the centenary celebrations of the railway later that year.
The design has proved itself in reliable service as one of the most powerful 15-inch gauge locos in the world. It is painted Muscat green with red, dark green and white lining after the style of the Highland Railway.
Two similar locomotives were built at Ravenglass for use in Japan. Northern Rock II was shipped in January 1990 and was followed by Cumbria in 1992.