In 1912, a single LNWR class G locomotive was fitted with a superheater and piston valves to evaluate if superheating was of use for goods service. As the outcome was positive, it was decided to rebuild further locomotives in the same way and designate them as class G1. Between 1912 and 1918, 277 further locomotives were either rebuilt from class B, or from classes C, D, E, F or G which had already been rebuilds from class A or B. The classes E and F which had been rebuilt to the wheel arrangement 2-8-0 again lost their leading axle. 

Between 1917 and 1934, the LNWR and the LMS built another 171 new. Later 320 were rebuilt to class G2A with a boiler that had a pressure of 175 instead of 160 psi. Some were even rebuilt back to G1 standard when they needed an overhaul. When British Railways were founded in 1948, 98 non-rebuilt were still in service.