Not a classic, but entertaining and different - Alistair MacLean, only its in the West, only it's mostly on a train and in the snow, only it stars Charles Bronson. He's a captured prisoner who winds up on a train headed to a fort where there's a diptheria outbreak... only he's not really a prisoner but a secret agent, and there's no outbreak, but a conspiracy between gun runners and Indians.
These twists might have been more exciting if we hadn't seen them in several MacLean works before - including an under-developed love subplot (Jill Ireland), a "surprise" traitor, a friend of the hero who is killed. Occasionally the film is let down by crappy 70s handling (those musical stings in particular always make it feel like a TV movie, as does - and this is a little mean but it's true - the presence in the cast of Ed Lauter).
But there's some great alpine scenery, the novelty of Bronson playing Miss Marple, I'm a sucker for a train movie, a sequence where a train falls of a bridge is genuinely spectacular, and the support cast includes Charles Durning, Richard Crenna and Ben Johnson.