Unusual for a North-American 4-8-0, the NdeM ordered their classes PR-7 and PR-8 mainly for passenger service. A thought behind this may have been that a Twelve-wheeler had more adhesive weight than a similarly sized Pacific. Their driver diameter of 67 inches or 1,702 mm was larger than that of any US 4-8-0 and matched that of the fast European locomotives of this wheel arrangement.

Baldwin built a single PR-7 in 1924 that was numbered 5-A and later renumbered to 2856. It had the same boiler as the KR-1 Mikados and the MR-6 Pacifics, but this was mounted higher in the PR-7 to make space for the wide firebox above the last set of drivers. Nevertheless, the top of the drivers protruded into the firebox. This was possible since this oil burner did not need a continuous ash pan. In contrast to its sisters with the same boiler, it had an Elesco feed water heater. The cylinders had square dimensions with a bore and stroke of 28 inches each.

Baldwin delivered five more in 1935 which had a larger boiler. These were called class PR-8 and numbered 3000 to 3004. The main increase in the firebox heating surface could be attributed to the thermic syphons. With a service weight of 288,000 pounds or 130.5 tonnes, they were the heaviest 4-8-0 locomotives ever. The dimensions of the cylinders and drivers had not been changed and also the boiler pressure remained unchanged. The PR-7 was retired in 1962 and the last PR-8 was gone in the following year.