TMOST mentions this I think - massive refrigeration facilities!
But the article is indeed a good one - it's fascinating to learn that they were intended as different locations. Was there ANY part of the set that wasn't redressed as something else as one point? Even the Bridge got this treatment (albeit to look like another bridge, but still...)
I haven't really thought about it, but I don't think the transporter room was ever anything other than the transporter room.