The various variants of the H6 formed the most numerous class of steam locomotives in the PRR. After the USATC S160, which had been developed for wartime use in Europe, they were also the Consolidations with the highest number of units. They were intended for freight trains on main and branch lines, local freight trains and as switchers.

The original H6 was built by Baldwin between 1899 and 1901 a total of 65 times. The firebox was narrow and long with a grate area of 33.33 square feet. These were not later retrofitted with superheaters and were retired in the mid-1920s.

On the H6a, the width of the firebox was increased from 40 to 65 7/8 inches, increasing the grate area to 49 square feet and increasing power. A total of 1,041 H6a were built up to 1905. Subsequently, 601 H6b were built, which had Walschaert valve gear instead of the Stephenson valve gear of the H6a. Since the PRR had influence in the Baltimore & Ohio at that time, identical locomotives were built for them and designated Class I-4.

In the 1920s, class H6a and H6b locomotives were equipped with superheaters and were thus designated H6sa and H6sb. Some locomotives got a cylinder diameter of 23 instead of 22 inches and a boiler pressure of only 195 instead of 205 psi, while others remained both values unchanged.

Today only number 2846 survives, which is in the Railroad Museum of Pennsylvania in Strasburg. In 1938 a total of 30 locomotives were sold to Manchuria and called “Sorisa” there, which came from “Consolidation”. They came to the Mantetsu and the Manchukuo National Railway. They later became part of the China Railway as the KD10.