These are really good motor blocks…Good performance IMHO…The 4 wheel version, are fitted into the RS3, U25B, ‘F’ units, Center Cab, and Lil’ Critter locos…The only differences between these locos and motor blocks, is the wire harness themselves…There are 2 versions of harness’…The first, has a total of 4 red quick connect plugs, 2 male and 2 female…These type plugs fit the ‘F’ units, U25B, and RS3 locomotives…The second, has 2 ‘pin type’ connectors, one, a two prong female receptacle, and the other, a three prong female receptacle (only 2 of the 3 receptacles is used)…These type connectors are used to plug into the Center Cab and Lil’ Critter locomotives…The motor blocks themselves ARE interchangable, and if needed, the wire harness can be swapped by opening the motor block, removing the small phillips head screws that attach the wireharness, swap harness’, replace phillips screws, re-assemble the motor block, and, VOILA!!!