The class M-2 consisted of twelve 4-6-6-4 “Challenger” articulated locomotives built in 1940 and 1941 by Baldwin. Designed for fast freight over the long climbs of the Western Maryland, they had drivers of 69 inches and four cylinders of 22 by 32 inches. The firebox had a surface of not less than 796 square feet or 74 m², thanks to three Duplex thermic syphons in the firebox and two more in the combustion chamber. On the drawbar they delivered around 4,700 hp. One of the steeper grades they were used on were 23 miles with a ruling grade of 1.75 percent on the line between Hagerstown and Connellsville. On downhill sections with 0.3 percent, they could haul 8,000-ton freight trains or 10,500-ton coal trains unassisted.

Their judgement by the crews was was highly controversial. Some called them slippery, underpowered or rough riding, but others praised their performance and running characteristics. Their service in fast freights with speeds of up to 50 mph ended in 1947. In this year, the 4-8-4 class J-1 was introduced that could haul similar trains with even higher speeds. Now the M-2 was mostly used as helper on steep grades. All were scrapped in 1953 when the Western Maryland started to dieselize its fleet.