Parallel to the K-6 with a two-cylinder compound engine, the Boston & Maine had ALCO in Schenectady build 34 class K-5a and b locomotives with a simple engine. They still had the Stephenson controls but piston valves. According to “Railway and Locomotive Engineering”, the dimensions of the heating surfaces were very well matched to the size of the cylinders. The locomotives later designated as K-5d were actually K-7s with superheaters.