In the first few episodes of TNG, the corridor to the left (from the point-of-view of facing the warp core) was open to the rest of the set. Trekcore has a set photo here showing the corridor open. A change was made in the middle of "Where No One Has Gone Before". Here's a screencap from early in the episode, when you can see the corridor to the left is still open: Here's one from later in the episode, when a console is added, similar to the one on the other side of the room, blocking the corridor: The console would remain for the rest of the series (if I'm not mistaken), only being removed when the engineering set was emptied to double as large corridor junction. Does anyone know why the change was made? Why did they put that console there? It's also notable that at some point (I don't know when?) the console on the right side was replaced with another door, as can be seen in this picture:
For context, here's the Season 1 floor plan of the entire set: (Of interest here, too, is that "sickbay" has six windows and the little corridor on the bottom-left. That's because during Season 1 the sickbay set doubled as the observation lounge. The main corridor would later be extended at the top to allow longer walking scenes from the holodeck/shuttlebay/brig set.)
Probably no way to know without talking to people who worked on the show, assuming they'd remember. Maybe they decided it made more sense for main engineering to be more enclosed rather than this walk-through space. Maybe they just thought it looked more interesting to have more viewscreens and gadgets in the background. Maybe it saved them the trouble of having to light the corridor.
Let us have a moment of silence for the missing former Chief Engineers MacDougal, Argyle, Logan and Lynch, who were permanently trapped behind the rapidly shifting hypercube of panels and corridors of the Enterprise-D's Main Engineering until the ship finally settled on La Forge as its one true Chief. Buried aliiiiiiiiive, buried aliiiiiiiiive!
You sir have the eye of an Eagle. I wouldn't have noticed in a million years (probably what they were hoping for). herman.zimmerman@paramoutpictures.com
It occurs to me that the sound quality might also be better in a more enclosed room than in the middle of a bunch of echoing corridors. But I'm not sure.
As far as I can tell, the set was just meant to be a corridor junction at first, with main engineering being the warp core and the office next to it. Then in Where No One Has Gone Before engineering expanded to take over the junction as well, with the curved walls separating the two areas removed, the table put in, and screens installed in the walls. The console blocking the hallway was just part of this new version of the set, and it was likely put in so that engineering didn't have a corridor running through it.
Yes, it was essentially a redress of the corridor outside the TMP engine room: http://pat.suwalski.net/film/st-stages/stage9.php
It is weird that they made the change mid-episode, but I think in the end it's a change for the better. To me, Engineering feels like one of those areas that should be a large space and/or one where you don't necessarily want random people just walking into the middle of things. IIRC the newly-added door is shown to be a Jeffries tube access point in later seasons?
I assumed it was done to make engineering look more like a separate set from the corridor junction. That was also often used with the table and the MSD removed and the corridor opened up again, it allowed for more variety in locations without every fan immediately noticing that it was a redressed set. This for example would have been much more obvious had the corridor and the door not been hidden with the wall plugs. The big door also wasn't a later addition, it was always part of the set, that's why the blue "walk here" carpet always ran into the wall even in the early episodes before the corridor was closed off. Edit: Looks like let added a different smaller door in front of the regular door as a later variation but that's still a wall plug and not a permanent change because the blue part of the carpet is too wide for the door and would have been changed if it was more than a temporary redress.
I'd say at a pure guess that it may very well have been down to budgetry constraints in the first season and only with the success of all of the subsequent seasons, maybe they had a little bit more money to spend on expanding that set. At least, that'd be my guess.
Except the change was made during the production of episode 4 of season 1. And it didn't expand the set, it technically shrank it. Since it was that early, it was probably just part of the initial feeling-out process. There definitely were a lot of set changes at the start of season 2, though. A new observation lounge set (no longer a redress of sickbay) and Ten Forward were built, sickbay was rebuilt, and the bridge was remodeled substantially. The bridge and senior officer quarters sets were also moved from Stage 6 to Stage 8.
Jörg Hillebrand and Bernd Schneider have tracked all the set changes between Seasons 1 and 2 at Ex Astris Scientia. Maybe there's something to the idea that, once they added the pool table, the set designers felt engineering had to become more like a room rather than an open space, as @Ray Hardgrit suggested. It's probably not a coincidence that these two elements were added at the same time?
That's the corridor from the "transporter accident" scene, isn't it? Meaning, after Kirk and Scotty hear "It's too late, they're beaming now!" they go running off down that same corridor, on their way to the transporter room.