After the Nickel Plate Road class H-5 of 1917, the next ten H-6a were USRA Light Mikados, delivered by ALCO. In the years 1920 to 1924, Lima delivered the classes H-6b to H-6f, which were virtually identical to each other and totaled 61 units. 15 more, which were again USRA Light Mikados, came to the NKP in 1923 via the acquisition of the Lake Erie & Western and became the class H-6o.
Over time, individual Mikados were equipped with mechanical stokers and larger tenders. The fireboxes were also changed. For most of them, the service life ended in the forties. Today 587, 624 and 639 are preserved, with the 587 having been operational for a longer period since 1988. 