We ran a pair of Fairbanks Morse OP 38 8 1/8 engines on NG (diesel start-diesel pilot) for years but that was for co-generating electricity. I don't know if they can be used for motive engines running NG.
Fairbanks Morse OP engines were used in the locomotive industry back in the 1950’s. Operationally, they were good powerful engines, but they were difficult to maintain. It is worth investigating. However, one thing that is not being discussed — railroads often loan power to other railroads. How would that affect the industry if the ability to use run-thru power from BNSF to NS, UP, etc, was no longer available?