I’ve been on the periphery of NG for tractor trailer stations. One thing that is a large part of the equation for truckers....and probably would be for locomotives also is maintenance. Just about anything that can go mechanically wrong with the engine can ultimately be traced to dirty oil...dirty as a result of combustion....and cleaner burning NG reduces maintenance costs.
Is the Diesel vs Crude decision also based in part on maintenance costs? I couldn’t imagine using crude as a fuel for anything other than a huge boiler.
BTW, I’ve spent a lot of time doing work at the BNSF shops, where they do engine overhauls, etc. It looks enormously expensive...there are swarms of union workers taking breaks and grilling steaks outside...just a giant sucking sound to watch the operation. I think it would be huge for the RR to burn a cleaner fuel, even if the costs were the same.
In my {limited} experience, cost always benefited diesel even without considering maintenance.
Keep in mind that crude oil has many components that are separated in the refinery to the highest paying portions. Burning crude oil direct forgoes that ability to maximize usage.
My experience with this was overseas. Here in the US, the sulfur content alone would not allow crude oil to be used as fuel for any permanent installation.