There were differences in locomotive control schemes that caused problems at first. Through the F3 model, the transitions were all manual. With the advent of the F7 model, transitions became automatic. Trying to use an automatic cab with a manual transition booster could cause problems because the older booster didn’t speak the control language of the newer cab. But eventually the older units were fitted with controls that were interchangeable.
Some FT units in later units were regeared and ballasted with concrete for use in drag service in combination with newer units. This was done because as they were originally built they couldn’t use their full horsepower at the slower speeds the newer units were capable of. But with regearing and extra ballast there was new life in old units. As long as the prime movers were in good shape life was good.
I have photos of all kinds of curious power combinations including rather amazing assortments of geeps with a cab unit booster thrown in the works for good measure. One I remember well was a photo of a GP40 coupled to a GP7, one unit being exactly twice the horsepower of the other. Again, the combinations were almost limitless and there were only a few combinations that couldn’t be made to work together. Baldwin units had pneumatically controlled MU interfaces and they couldn’t be used with those that had electrical interfaces, so that would be a no-no unless the Baldwins were used as a tail end pusher or something like that. But otherwise, the only rule was if it works for you, try it. I find it fascinating to see the interesting combinations that appeared over the years and I like to model these in my own private world.
One combination on the Union Pacific that got quite a bit of useage was putting an E8 or E9 A unit with a F9 B unit to make a locomotive with a total of 3,900 horsepower. Union Pacific used a lot of these combinations in freight service after the demise of long-haul passenger service. They were very effective. There was