Deliberate mistake: The entire plot wouldn't exist without this one, but just before the engineer stumbles out the door and falls off, he pulls the big red lever on the left side of the cab. In the real world this puts the air brake system into its emergency mode. Among other things, this mode trips a solenoid which causes the engine to return to idle, and the electrical system to not put out power. Neither the Diesel engine nor the electrical system will respond to control inputs until/unless the air system is reset.

Runaway Train (1985)
1 deliberate mistake
Directed by: Andrey Konchalovskiy
Starring: Jon Voight, Eric Roberts, Rebecca De Mornay, Kyle T. Heffner
Continuity mistake: After Manny's fingers get crunched between the cars, blood sprays on his face. In the next couple of scenes there is hardly any there. Also, when the prison warden get sprayed with the fire extinquisher he is cleaned up in the following shots.
Rankin: Push the button. We're on a dead-end siding. We're gonna crash in five minutes.
Oscar "Manny" Manheim: Then we'll have a nice, five-minute ride together.
Rankin: You think you're a hero, huh? Shit. You're scum.
Oscar "Manny" Manheim: We're both scum, brother.
Trivia: In the original script Manny was supposed to be a convicted killer but writer Edward Bunker changed it to a safe-cracker because he didn't feel the other prisoners would respect a killer.
Question: Why didn't they go back to the 3rd or 4th locomotive where they could access the brake hoses and disconnect one of those, instead of trying to get to the hoses between the 1st and 2nd engines?
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Answer: That's what they were doing, going car to freight car break the hoses. They knew it would slow the train down but not stop it. Slow enough so they could jump off without getting injured. That's why they were desperate to get to the first engine not only to break the hose but disconnect from it.