Question: How did Doc originally plan to get from the train and into the passenger seat of the DeLorean? The tires didn't look very secure, so stepping on them would've likely resulted in them falling out of the carrier. Even if by some chance he had made it onto the back of the car, he'd then have to avoid damaging anything like Mr Fusion, etc., as he made his way to the passenger door.
Question: Doc is a scientist right? The DeLorean had a ruptured fuel line and needed gas which, as Doc pointed out, was unavailable at the time. Surely he knew how to distill booze to make ethanol? (There was plenty of whiskey around at the time). I mean, they've used it to power cars in Mexico for ages. Why didn't Doc suggest this?
Answer: They did try it. Doc ran the engine with the strongest thing the bartender could find them and it blew out the engine. It takes all the power a heavy car like a DeLorean has to get up to 88 MPH. It wouldn't be able to get that much power running on ethanol, in addition to the damage caused to the engine.
Answer: Putting to the side for a second the possibility that he either did not know how to do this, or simply didn't think of it... a quick Google search says that the entire process to produce ethanol would take about a week. They had about 3 days because they were trying to leave before Doc got killed.
Answer: He presumably wanted to jump atop the DeLorean.