In cases where railroads have engines sidelined awaiting restoration such as FEC #148, Gettysburg #76, CP #972 and RDG (#4) #1187, PRR #1361 and various other stripped down boilers around the country and around the world, sitting outside exposed to wind, rain, snow and other elements, many of them display an awesome coating of rust. Providing said company decides, "...Okay. Now we have the money, lets get 'Old Betsy' back on her feet again..." how would all that rust be removed?
As an ex-fireman, (Volunteer/Municipality not loco fireman) I have learned in fire school that fire is the rapid oxidation of a fuel accompanied by the release of energy in the form of heat and light. Or something to that effect (Its been over 12 years since I studied Fire I.)

Slow oxidation is rust.

Over time such as engines like the aforementioned 76, 148, 972 and 1361 whose boiler jackets have been long removed and asbestos properly disposed of ages ago sit without any protection. What type of damage does this particular kind rust do to the boilers? Would there be any degradation in structural integrity or would the boiler hold up to an ultrasound once mapped and if so, what kind of rust removal techniques are used to make the metal gleam once again? Thousands of gallons of WD-40 would probably be ruled out at this point I would imagine.

The old saying an once of prevention is worth a pound of cure applies here. A simple and even poorly applied coat of paint goes a long way towards preventing corrosion (rust).

Metal is cleaned of corrosion by a mechanical means, often some sort of blasting (sand or other media) or by a powered wire wheel. Down under all of that oxidization you will find shinny clean metal.

In preservation it's important to know that there are several metal related issues we face; corrosion, fatigue, pitting, and scale. Each has it own remedies, which often means cutting out and replacing the metal.

Jackets and lagging actually hold water and nasties in solution - like H2SO4, if I remember my chemistry right - and contribute to corrosion. Free air circulation is much preferred. Once the loose rust is removed, the surface patina can be neutralized with a rust converter, which seriously slows further oxidation. There's not a lot on a locomotive that actually needs to be shiny.

dave