Some of the best info for what would be required for treating saltwater-immersed locomotives could be obtained from the CSS Hunley Confederate submarine project:

http://www.hunley.org/

The sub was raised from the harbor in Charleston in August 2000, where it had lain since it sunk in 1864. Archaelogists concluded that the sub had been fairly rapidly covered by a silt layer which provided an anerobic environment that greatly slowed deterioration of the hull. The sub was in remarkably good shape considering it had lain in relatively shallow, warm salt water for 136 years.

A large rectangular tank was constructed to house the sub so that it could be kept immersed in cold, fresh water while they figured out how best to treat it. It was mentioned that part of the problem was the sub was constructed of several different metals including wrought iron, cast iron, bronze, and probably others. A treatment process that would work well for one metal might prove disastrous to another.

The sub was recovered over 11 years ago and AFAIK is still kept submerged most of the time, although the tank has been drained repeatedly to allow access by archaeologists. The hull was very recently rotated to an upright position for the first time since it sunk in 1864.

I don't know what the current state the treatment process, but I'd guess these folks know as much about what needs to be done to preserve recovered large metal objects as anyone in the world.