Between 1904 and 1908, Webb's successor George Whale added a leading pony truck to 26 engines, making them 2-8-0s and taking them into Class E (again from 1911).

Between 1906 and 1908, Whale also rebuilt 10 with larger Experiment-type boiler to Class F, again adding a leading pony truck.

Neither of the above conversions was particularly successful and, as a result, 32 were rebuilt to Class G with 2-cylinder simple expansion between 1910 and 1917.

The London, Midland and Scottish Railway (LMS) inherited 53 unrebuilt Class B locomotives in 1923 and numbered them 8900–8952.

The LMS continued to rebuild them to Class G1 and the number of unrebuilt locomotives dwindled to 10.