It was a class of heavy mixed-traffic locomotives of the 4-8-2 "Mountain" arrangement, which uses four pairs of driving wheels with a four-wheel guiding truck in front for stability at speed and a two-wheel trailing truck to support the large firebox needed for sustained power.

The M1a locomotives were intended for passenger as well as freight service, and some bore decorative gold-leaf lining on tender and cab sides, but they proved better suited to freight work, and extensive electrification saw a surplus of K4s locomotives available for passenger trains.

While they were mostly used on freight, their dual service purpose entitled them to have smokebox mounted keystone shaped numberplates.

40 locomotives were later converted into class M1b; the differences were all in the boiler, those being the addition of firebox circulators—large tubes carrying water passing through the firebox space, increasing water circulation and heating area, and thus steam generation—and an increase in boiler pressure to 270 psi (1.86 MPa).

The only externally visible difference was extra cleaning plugs in the firebox sides, for washing out the circulators.

After World War II, the M1 locomotives underwent similar front-end changes to those made to the K4s, including sheet steel drop-coupler pilots, the moving of the headlight to the smokebox top in front of the stack, the moving of the steam-driven generator to the smokebox front for easier maintenance, and the addition of a wide step above the air tank to service the air compressor.

One locomotive, M1b #6755, was saved from the scrapper's torch for the PRR's historical collection housed at Northumberland, Pennsylvania.

The tender of M1 #6659, minus the engine, has also been preserved and was purchased by the Pennsylvania Railroad T1 Steam Locomotive Trust in August, 2017.