The climactic train collision and gang-fight depicted in this episode was actually old stock footage borrowed from the film Saratoga Trunk (1945). In fact, this would explain why Jack Kelly spends the whole episode wearing a large white Stetson hat, as opposed to his usual smaller black hat. During the gang-fight, there are shots of Gary Cooper, "Saratoga Trunk's" star, from behind, so that a viewer cannot see his face. Thus, through clever film editing, "Maverick's" audience is intended to believe that it's Kelly under that white Stetson, and not Cooper.
Wilhelm scream heard during fight as man falls from side of train (about 42:15).