While I don't claim to be up to speed on locomotive turbochargers, I have worked on turbocharged aircraft engines (the reciprocating variety) for the past thirty years, and I would think the two are quite similar in both function and operation.
Bear in mind the aircraft engine has fewer cubic inches per engine than the prime mover has per cylinder, and a much smaller turbocharger, but is capable of maintaining sea level manifold pressure, and full horsepower to VERY high altitudes. I can't imagine diesels giving up that power at altitudes below 10,000 feet. There is a controller that prevents over-boosting the engine and causing internal damage. I would tend to think all engines have a similar setup. Pilots tend to have a heavy hand on the throttle, and the controller is the only thing that saves their bacon.
If locomotive turbos are set up like aircraft engines they should be able to maintain full power at all elevations.
I may be wrong, but those are my thoughts.
