Wasn't it a model for the crash and a full size mock up for the scene where it had come to rest in the sand?
Actually three different 4449 "pieces" where used in that famous "crash through the end of track" scene:
First, the real 4449 ran through the "border/end of track" scene staged on the Eagle Mountain RR, which had been rented for the movie filming.
Second, a 1" scale , live steam model was used for the scene where she "buries herself into the sand" (actually I think they used ground-up corn flakes), as if it was being photographed from a chopper.
Third, there was a full size, plywood mock-up of just the Engineer's side of 4449, buried into the desert sand. That is the scene where Burt & Kirk step out of the cab and walk away, only to meet the Mexican authorities.
