Engineer (Bill McKinney): "How fast can she go? Why, I've had her up to 55, myself. I heard that Fearless Frank Fargo got one of these near 70 out past Verde Junction."
Marty: "You think it's possible to get it up to 90?"
Engineer: "Ha! 90? Tarnation, son, who'd ever need to be in such a hurry?"
Doc: "Just a little bet he and I have, that's all. Theoretically speaking, could it be done?"
Engineer: "Well, I suppose if you had a straight strech of track with a level grade and you weren't haulin' no cars behind you, and if you could get the fire hot enough, and I'm talkin' about hotter than the blazes of hell and damnation itself, then yes, sir. It might be possible to get her up that fast."