Just out of curiosity, since I've already solved my problem by replaying from a previous save....
In Rhem 3 there's a corridor that sequentially lights up when you enter it and has an elevator at the end. I went down the ladder that's concealed by the elevator and did quite a bit of exploring in this area of the game. Later, I returned to the bottom of the concealed ladder and lowered the elevator. I then disabled the "open" code on the door, for some strange reason, went through this door and explored some more. When I got back I realized that I had made it impossible to get back through the door with the "open" code because the button to open it is hidden by the elevator shroud. Is there another way, besides this doorway, to get back up to the surface where the tram is?
Did you ever stop to think and then forget to start again?