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 2032 units. The three sub-classes differed as follows:In the 1920s, 699 H6a and H6b had superheaters added and cylinder size increased from 22” to 23” and reclassified to H6sa and H6sb.Class H6 were used throughout the system as mainline freight haulers, on local freights, and as switchers in yards.