Stagecoach robber Ben Wade (Ford) is caught, having made the fatal mistake of dallying with a barmaid after a raid. Since the captors know his gang will be coming to free him, they run a decoy off, and instead hire ranch-owner Dan Evans (Heflin) to take Wade to Contention City, and await the titular train. But Evans' farm is in trouble, and Wade realises that gives him leverage: can he convert it before he gets shipped off to face justice? Based on a short story by Elmore Leonard, this starts sluggishly: Chris was muttering disapproving comments under her breath early on, and we were highly unimpressed by a theme, sung by Frankie Laine, which rhymed "Yuma" with "rumour" (fortunately, it stopped short of throwing in "puma" or "tumour"). However, by the end, Chris had been won over, and had to admit this fully justifies its reputation as a classic, and it succeeds in creating tension at a level not often seen in the genre.
I think it's probably the performances which make it work. Ford and Heflin are both excellent: Ford exudes a charming menace, while Heflin does a great job of putting over a character who has, initially, no interest in getting involved, and has, basically, to be bribed. Gradually, he finds a moral spine, as those about him are losing those, and Wade finds himself drawn to respect a man who won't quit, though it would be easy - and profitable - for him to do so. It's a fascinating moral study, and the jousting between the two men makes for great viewing. Those expecting an explosion of...well, explosions, or major gun-play, as in the remake, may be somewhat disappointed, but I'll be impressed if the new version comes anywhere close to the standard of the original.