It was a superheated version of the LNWR Class G with 8 inch piston valves.

[1] The prototype was rebuilt in 1912 from a member of Class G and a further 170 new locomotives were built between 1912 and 1918.

The London, Midland and Scottish Railway (LMS) renumbered the engines into a more logical series.

However, they also then continued to rebuild engines, which retained the numbers originally assigned by the LMS.

British Railways (BR) inherited 98 locomotives in 1948 and numbered them in the range 48892-49384.