Breakheart Pass by Alistair MacLean

The Rocky Mountains, Winter 1873 . . .

One of the msot desolate stretches of railroad in the West. Travelling along it is a crowded troop traiin, bound for the cholera-stricken garrison at Fort Humboldt. On board -- the Governor of Nevada, the daughter of the fort's commander and a U.S. marshal escorting a notorious outlaw. Between them and safety are the hostile Paiute Indians -- and a man who will stop at nothing -- even murder . . .