I think, and this is just a guess, that those big ship motors can idle at really low rpm. At low rpm the prop is low(er) drag. Or maybe it has a clutch? zero idea. But a train has to to operate down to 0mph. Water has slip, trail rails don't (or shouldn't, not desirable).Since this thread has sort of morphed into a discussion of hybrids and EVs, how come something like a diesel electric system like locomotives and cruise ships hasn’t been tried? 100% electric propulsion powered by an onboard diesel generator. There has to be some advantage to a system like this or it wouldn’t be so common.
The locomotive is kinda stuck with electric as it'd take a lot of gear reduction to get started, then a bunch of shifting. The cruise ship I do believe is more like an automatic transmission setup, the prop&water is like a torque convertor setup.
Maybe a train could eek out more miles per gallon (or less gallons per mile?) if it could have some top gear direct drive, but it's the transmission that is the issue.