As the article and the further details in the linked report, this is for a cost savings.
These engines can be slightly modified to run on crude oil also.
Which is still more expensive than LNG. Crude oil engines are not an economic choice if more refined fuel is available. I've participated in cost comparisons for oil field applications. For the biggest one, it was cheaper to build our own processing unit to make our own diesel than it was to burn the crude oil.
It is my understanding that crude must be heated to boil out the water and filtered. Some modifications might nave to be made on injectors and injection pumps. This modification seems cheap and small when compared to a high pressure refrigerated system required for natural gas. The diesel part would have to be maintained for ignition of the gas in the engine or a spark system would have to be added to the engine. Then there is a whole new infrastructure to deliver the natural gas also.
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.