Counterproposition to Whorfin
: TOS and TMP arrangements were basically identical, but in TOS we saw standard operation and maintenance procedures which could be handled from protected locations. In TMP, we saw engineers working in spaces that were not "main engineering", but rather the "inner sanctum", because the ship was still in the process of being built and tested. The real Main Engineering of that ship was a room not unlike the TOS one, with tidy control consoles far away from the glowy bits, requiring no radsuits or other protective measures. It just wasn't featured in the movie.
In ST2:TWoK, we again saw the interiors because this was a training cruise where emergencies and nonstandard procedures as well as the deepest inner workings of the ship were the target of training. In normal ops, Scotty would rarely don a radsuit and enter these spaces.
Possibly the real Main Engineering was seen in ST6:TUC, featuring just a single thickly protected section of the vertical power conduit, or perhaps the actual reactor part. (That is, the barely glimpsed TNG set reuse was the shirtleeves part of the TMP setup.)