After Baldwin's first Consolidation locomotives were not well received by American railroads, this wheel arrangement had a breakthrough in 1875 when the PRR made it its standard freight locomotive. It started with Class I, which later became Class H1. A total of 545 of these were produced in the PRR's own factories in Altoona.

Curve negotiation was guaranteed by the fact that only the first and last driving axles had wheel flanges. The special thing about the firebox was that its ceiling sloped backwards. However, this arrangement made it difficult for the crews to set the correct water level in the boiler during operation. The later H2 thus received the Belpaire firebox, which was soon used as standard on the PRR.

Gary Rauch / prr.railfan.net

Despite the difficulties with the firebox, these locomotives regularly pulled 80 to 90-car freight trains at 14 mph. Because of their pulling power, they were also called “Modocs” after the Modoc Indians. Later, 57 were converted to class B5 with a 4-6-0 wheel arrangement.