The Pennsylvania Railroad's class H6, H6a, and H6b steam locomotives were of the 2-8-0 "Consolidation" freight type, the most numerous class on the railroad with 1,707 units and the second most prolific 2-8-0 class in North America, with the USATC S160 class rostering 88 units more. The three subclasses differed as follows: In the 1920s, 699 H6a and H6b had superheaters added, and cylinder size increased from 22 in (560 mm) to 23 in (580 mm). These rebuilt units were reclassified to H6sa and H6sb.