Well on trains they use 2 stroke diesels, or at least they did up to 2006. And 2 strokes run in that range of efficiency
and it don't matter if you use a gearbox, gears multiply torque, they don't multiply horsepower.
The power into those generators even geared is the single most efficienct use of a diesel engine.
Get the right gearing, spin the generator at its required speed, use an isochronos governor. ( look that up!)
and bamm super efficiency