A number of railroads routinely m-ued F units and geeps. In general, FT's would not m-u with geeps, or even later F-units (except F2's), because of a different wiring configuration on the m-u receptacle. Santa Fe had a few special jumper cables with one end having the standard 27-pin plug and the other end fitting the FT. Rock Island is the only carrier I can recall which routinely m-ued FT's with other models, and they might have modernized the m-u setup on their FT's and F2's. However, in 1961, I watched a Denver & Rio Grande Western passenger train depart Salt Lake City behind a consist of FTA-FTB-F3B-F3A. I suspect they used a special jumper cable between the FT's and F3's.
Even GP30, GP35, SD40, and SD45 units built through the late 1960's still had a transition lever on the control stand, in case there was an F3 or other manual-transition locomotive trailing in the consist.
