As a kid I used to see EMD GP9/7 M.Ued with F units ( both A & B units ) all the time on the B&O's Washington Branch ( now CSX/ MARC Camden line ). Chopped nosed hood units were in the locomotive consists at times too. Occasionally I would see an end cab switcher in the loco consist as a trailing unit.
On this line B&O ran E units on their passenger trains back in the 1960s. They ran one F unit on some of their commuter trains while other commuters were pulled by a GP9. I never saw a mixed locomotive consist on a B&O passenger train on this line .... although B&O may have done it and I just never saw it. RDC's were a common commuter train on this line as well. I'm sure B&O mixed passenger train locomotive consists at time elsewhere on their system though.
The Western Maryland would also put together locomotive consists ( coming in /out of Baltimore running west and east ) using A&B F units with GP units and perhaps some ALCO FA2 at times. Sometimes I'd see long freights with mid train helpers of F units ( usually two ) coming out of Baltimore. I would see these trains as the WM main line ran under Liberty Heights Ave. in Baltimore City. A great place to view WM trains back around 1970!
I love the eclectic look of the mixed locomotive consist comprised of first generation diesels ... and also love first and second generation diesel consists as well! To me, the look conveys a sense of ultra power and ruggedness ... even though under the hood the units are basically all of similar horsepower. As a modeler these mixed consists also provide a nice visual contrast to the all F or E unit ABBA consist. After viewing several mixed consists, my eye has a greater appreciation for the sleekness of the all F or E unit consist and/ or the all PA or FA consist which allow these sleek consists the streamlined look of which they were intended.
Most enjoyable thread! I've enjoyed reading everyone's comments!