After I got the sewer pipe rerouted and the sewer drain pump running, the light turns green to show that the flood door will open now, and I can now start the little train engine.

I drove the train engine towards the door, and the door opened automatically. Interesting.

But rather than go through, let's stop the train engine, shift it to reverse and back away. When I get far enough away the flood door closes.

Save game, run around map to see if I missed anything. Go back to the train engine, shift to forward, and this time as I approach the door...... it doesn't open, the engine goes straight through the closed door.....

https://imgur.com/OXv8iY2

.... and when I reach wall, I am pushed off the back of the train onto the floor as it goes right through the closed door and disappears. Where'd my train go???

https://imgur.com/CjlMbD7

I previously read that someone got past this by standing in front of the train as it moves forward and it shoved them through the closed door. This does not work for me. Sure, the next map loads but now I am stuck "inside" the door behind the train and I can not move.

https://imgur.com/Ktn3kNq

So now I can't get out of this as I started from the very beginning, using a single save slot, and I saved over the previous "good save" before all this happened, so now I am stuck in a saved game with a screwed-up flood door that won't open for me a second time.