They've saved the best trip for last... But this time they may have gone too far.
Why it is hard sci-fi
A western that quietly does engineering. The DeLorean cannot run, so Doc and Marty rebuild a steam locomotive into a launch platform. Pre-calculated track segments, ignition wood for higher burn temperatures, and a fixed take-off speed of 88 mph. The film treats the problem as a budget of energy and distance.
Science inside it
Steam thermodynamics. Pres-tolite logs are introduced as a way to drive boiler temperature high enough to push the train past 88 mph in a fixed distance. The film names the variables and then runs the experiment.