The units had a 1,200-horsepower (890 kW), six-cylinder opposed piston engine prime mover, and were configured in a B-B wheel arrangement mounted atop a pair of two-axle AAR Type-A switcher trucks, with all axles powered and geared for a top speed of 60 miles per hour (97 km/h).

H-12-44s were visually indistinguishable from the predecessor FM H-10-44 until September 1952, when the Raymond Loewy design elements were removed to reduce production costs.

Cab lines were squared-off, the slanted-nose styling was discontinued, and the roof visor was eliminated.

Production paused from May to October 1956, after which the carbodies were shortened by some three feet and outfitted with a deeper side skirt.

Sixteen intact examples of the H-12-44 are known to survive, all of which are owned by railroad museums or historical societies.