[Marty and Doc are asking how fast the train could go]
Marty McFly: Do 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: Well, it's 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 stretch of track with a level grade, and you weren't haulin' no cars behind you, and if you can get the fire hot enough, and I'm talkin' about hotter than the blazes of hell and damnation itself... then yes, it might be possible to get her up that fast.