I think once Doc discovered time travel he kinda of knew what was going to happen. He also was part of the past! BTTF3! Kinda like Bruce Willis in The Movie Looper! I think Doc discovers that he only has to change the time on the DeLorean when Marty 1 comes back from 1955 and sees Marty 2 go back in time. That becomes the main timeline anyway after the events that happened. By just having him return at the same time he left. Maybe he tells Marty this once Doc arrives with The time traveling train. And we just don't see it. Since to me that seems to be a fix point in the time line. That closes that loop.