The Class M was the first variant in a series of Rhymney Railway tank locomotives that led to the class 5600 of the GWR. It had been developed by Richard Jenkins and had three driving axles with relatively small wheels in order to achieve high tractive power for the heavy coal trains. Although some successor class P and A engines were later rebuilt to a similar level as the 5600, only one class M loco was so rebuilt. All came to the GWR in 1923 and British Railways received three in 1948, which were retired by 1951. 