This is a sketch of the Tartarus Federation machine room set from Reunion. From this image, you can see the set was planned as a series of walkways and tubes. This is pretty much how the final set turned out. One thing I could not spot in the final episode are the two doors evident on this sketch. You can see from the written notes, they planned to reuse parts of the train from Destination Terminal to help make up the set. I did not see any parts of the Omni-Pacific train in this set, but later in the episode at the research station there is a clear reuse of the doors from the train.
The final set in the episode is a little different. For example, there are stairs that are not on the original sketch
