The Rhymney Railway M class was a class of 0-6-2T tank locomotive introduced into traffic on the Rhymney Railway in 1904. These were substantial sized tank engines, and weighed 66 long tons (74 short tons; 67 t) (64 long tons or 72 short tons or 65 tonnes after rebuilding) and were 36 feet 9 inches (11.20 m) in length. There were six locos in the class. They were built by Robert Stephenson and Company and are sometimes referred to as the Rhymney Stephensons even though Hudswell Clarke and Beyer, Peacock and Company contributed many of the derived designs.