Bob,
Paint? So, what's to paint. Fabricate the interior framework of the car (Chama's cars are a steel tube framework) and cover with the fiberglass panels currently in use on the exterior of the Chama cars (not sure what the Antonito cars are covered with). Aircraft primer (zinc chromate) should be sufficient to inhibit corrosion on the framework. Attach the outside panels with aluminum rivets. Now welding up the framework (and later repairing it) is another story. That will require some training, no doubt in Mig welding, but the local welding supplier (like Lincoln electric can provide that training.) It all sounds so easy, bu-u-t, being in maintenance management I know how Murphy's law works.
Sigh, a 28 car double-headed passenger train attacking the Chama hill with a vengence, with El Coke and Earl in the hoggers' seats. I would pay triple for that ride!!! Gees, I hope I can make it out for a September visit.
Mark V