How Doc built a flying, time traveling train in 1885
https://www.reddit.com/r/FanTheories/comments/66kaoh/back_to_the_future_how_doc_built_a_flying_time/
He didn't do it in 1885, he was brought to the future by a past version of himself.share
During the time between Part 1 and Part 2 Doc is off on his own time traveling adventures. We know he went to 2015 and bounced around a bit, getting the hover-conversion, change of blood, checking out future editions of USA Today, but it's never confirmed how long he spent time traveling between the events of Part 1 and Part 2. With Mr. Fusion there is essentially no limit to the number of trips he can make.
I propose a theory: During this time Doc traveled to Hill Valley on the same date every year, going back potentially hundreds of years, and stayed in a certain spot for say, an hour, as a fail safe for himself.
If he ever became trapped in time he just needed to meet up with himself, and did so in 1885. "Past Doc", the one experiencing events before BTTF Part 2, brings "Future Doc", the one who was left in 1885, and Clara to the future to get the tech they need to create a flying, time traveling train.