I was thinking about BTTF3 and it occurred to me there was an easy solution to their problem. What 1885 Doc should have done was to leave himself a note with the cave car, telling 55 Doc what happened when Marty travelled back to 1885. With the knowledge of the car being damaged upon arriving in 1885, 55 Doc would make sure to pack a spare fuel line and a jerrycan of gas. Assuming the damage was unavoidable, 1885 Doc would have the parts and gas needed for the return trip, so no need for the train.