Chris MacDermot wrote quite a bit about this subject in his book of Alco tips, "The Service Engineer". While 27 pin MU may seem like it is the same for Alco, EMD and GE, it really isn't the same once in the electrical cabinet.
All of this is from memory, as I don't have the book handy, but- There have been cases where an Alco will lead an EMD, and because of the way the generator field (6) and reversing circuits (8 and 9) are handled, the EMD winds up with chewed-up SP contactor tips.
An EMD leading an Alco will sometimes cause the Alco reverser to shift under a light load ("light" due to residual magnetism in the generator field) and the Alco reverser tips will suffer. And so on; several other examples of MU incompatability are cited.
What does this have to do with GEs? Well, no two GE models seem to be alike. While Alco and GE use a lot of common equipment, they are often wired differently. Some of the GEs are wired similar to EMDs, others are more like Alcos.
Chris suggests some wiring changes to be made on 244 and 251 Alcos, to make them "EMD compatable". He also suggests that any older GE (U-boat vintage) be studied and similar changes be made, if needed, on a case by case basis.
I asked Chris when he came up with these fixes; he told me that he and Norm Anderson figured out most of them when the PA-4s were at M-K, back in 1975.
So, if you see a photo of NKP (LV, PRR, etc.) RS-11s and Geeps in MU together, someone is doing some damage in someone elses electrical cabinet.
Needless to say, "The Service Engineer" is a "must have" book for anyone that still runs Alcos, with or without mixing them up with other makes.
