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Diesel-electric locomotives could very easily run on NG, because diesel engines can use it directly so long as you don’t make the fuel-air mixture rich enough to auto ignite. You just use a pilot charge of diesel oil to trigger ignition. A thermodynamically very efficient use of fuel, especially desirable when gas is so relatively cheap.

Hmmmm.....interesting info. I wasn't aware that could be done. Gotta read up on that....thanks!

Makes the potential speed and breadth of application MUCH wider.

My source is recollection from my textbook from college half a century ago. Running a diesel engine on natural gas as described above was done in Alaska because NG was much cheaper locally than diesel oil - back before that book, titled IIRC “Internal Combustion Engines,” was written. The text described the result as follows: “ . . . may be the most efficient prime mover in existence.” The text also said that the combustion of the lean NG-air mixture in a diesel engine was smoother than the notorious “diesel knock” of the same engine running on oil.
The textbook indicated that the fuel-air ratio could be modulated to control the power output, but my suspicion was that load control was needed to make it really practical. That is, IMHO you really need to control the power output by controlling the RPM of the engine (power output for a diesel being roughly proportional to RPM). That’s pretty much impossible with a conventional mechanical transmission with a finite, and small, number of gear ratios. You would need either a continuously variable transmission (which in general has not been available as a practical matter) or to use a generator to convert the power from mechanical to electrical, and electric motors to accomplish the CVT function. And I think that is precisely what the diesel-electric locomotive actually is (it’s certainly what a “hybrid” automobile is - with the added feature of battery energy storage, which makes the problem of engine power control even less of an issue. Since the hybrid auto places fuel economy above initial-cost considerations, and since engine RPM is independent of vehicle speed in any hybrid, I’m a a loss as to why they don’t go whole hog and power hybrid cars with diesel engines to get even better fuel efficiency).