Why Did Doc Take The Tracks?

In Back to the Future 3, Doc goes back to 1985 via a flying train, if the train flies, why did he go on the tracks. If a 1985 train was coming, this could have resulted in serious injury to himself, his kids, and his wife. Marty almost got crushed moments earlier, what if he came out of time and crushed Marty? I understand he may not want to be seen, but he flew off into the sky at the end anyway? Why take the risk?