The good experiences with the first narrow-gauge Garratts led the Tasmanian Government Railways to order two Cape gauge Garratts each for passenger and freight trains in 1912. Two class M Double Atlantics were ordered for passenger service, which had four cylinders of the same size in each bogie. They were the only Garratts ever built with eight cylinders.

They used the same boiler as the class L freight locomotives. They were probably the most powerful articulated passenger locomotives in the world at the time of their entry into service and often reached speeds of 50 mph or 80 km/h in daily use. The top speed was about 60 mph or 97 km/h. Since the eight cylinders were very time consuming to maintain, they were phased out in the mid-1920s after the class R Pacifics had been introduced.