I recall that on PRR, the Baldwin sharks would not MU with anything other than another shark due to the air throttle.
On NYC, the railroad at first tried to keep like brands within a territory. For example, up to the U25B era, they tried to keep all Alcos east of Buffalo and all EMDs west of Buffalo. The main maintenance point for all EMD power on the mainline was Collinwood, and for Alcos it was West Albany for overhauls and Dewitt (Syracuse) for running maintenance. Off brands found their way to the Big Four. Larger fuel tanks and the goal to obtain greater monthly mileages (i.e utilization) resulted in run throughs that resulted in a broad mix of EMD, Alco, and GE, at least on the mainline. There were problems in making up MU consists and this led to the installation of nose MU receptacles, so a covered wagon "A" unit could be in the MU consist and did not have to lead.
I also recall a fellow GE employee and ex Rock Island guy who told me that he sat down with the RI electrical guy, and they made up a chart of what units would MU with what other units. They found six that not only would NOT MU with anything else, they would not MU with each other!
