The Mallet compound, which re-uses steam from the rear high pressure cylinders in the larger low pressure cylinders in front, was common enough on railroads that operated in mountainous territory, including many eastern carriers. However, locomotives of this type were absent from the Pennsylvania Railroad roster until World War II when a surge of traffic created the need for additional power. In 1943 the PRR bought six surplus class Y-3 2-8-8-2s from the Norfolk & Western, including No. 374 (the former N&W 2008), designating them as class HH-1. Unusual for older work-a-day Pennsy freight engines, No. 374 sports the keystone number plate on the smokebox front instead of the round plate.

The N&W Y-3 class came from both Baldwin Locomotive Works and the American Locomotive Company in 1911, the above member being an ALCo product. They weighed 539,000 pounds and mustered a tractive effort of 114,148 pounds. Their high-pressure cylinders measured 25x32 inches, the low-pressure cylinders having a 39-inch diameter. Rolling on 57-inch drivers, they developed 270 p.s.i. of boiler pressure. Evaporative heating surface totaled 5753 square feet, with 1582 square feet of superheating surface and a 96-square-foot grate area. These ponderous behemoths enjoyed only a few years of service on the PRR before being retired between 1947 and 1949. William D. Volkmer provided the image.