Most of the H-5 class Mikados came from the rebuilds of a total of 482 G-5 class Consolidations. Not only were locomotives converted directly from the New York Central, but also by its subsidiaries Boston & Albany and Big Four. The firebox remained largely unchanged and the boiler barrel was lengthened. While the cylinders were enlarged, the boiler pressure was reduced by 20psi. The combination of these measures allowed to increase speed. Overall, the adhesive weight also increased. After the G-5 had already been built in different versions, there were also many differences within the H-5 class. In addition to the converted locomotives, 179 were newly built.