: The central mystery hinges on the fact that nothing is as it seems—the "epidemic" is a ruse, and the "heroic" soldiers on board are often part of the corruption.

The plot revolves around a relief train supposedly delivering medical supplies for a diphtheria epidemic at Fort Humboldt. In reality, the train is a vessel for a treacherous conspiracy involving stolen weapons and gunrunning.

: Charles Bronson's character, John Deakin, is introduced as a mysterious outlaw but is revealed to be a federal agent. Unlike typical Bronson roles defined by "kick-ass" physical prowess, Deakin must rely on deduction and observation to survive a train where almost every passenger is a potential conspirator.

At its core, Breakheart Pass is a subversion of the Western hero's usual "might makes right" philosophy. While it features classic elements—train shootouts, dynamite, and "marauding Indians"—it is frequently compared to .