The engineering section of the refit Enterprise is supposed to be lodged on Decks 14 and 15, with controls and the horizontal intermix chamber on the lower level. A dilithium reactor room was added to the port side of main engineering in Star Trek II: The Wrath of Khan.
Production Designer Harold Michelson was responsible for giving the engineering set, which had already been built for the aborted second Star Trek television series, Phase II, a grander scale in The Motion Picture.
“We wanted to give the ship a new kind of engine, something never seen before,” he told Starlog magazine (January 1980).
It had to suggest limitless power. We built it three stories tall; and it was as if it extended all the way down one of the nacelles.
He is referring, of course, to the engineering hull.
“The engine was a Plexiglas thing,” Michelson explained to Star Trek: The Magazine in 2001, “and I assumed that it would go all the way back through the ship and up through the two diagonal supports to the nacelles.”
I did it forced perspective so that I had a five foot man, a four foot man and a three foot kid standing on the set to create the illusion of depth. We were able to send this energy through that thing and up the tubes and it worked out very well.
Michelson told Fantastic Films 14 (February 1980, My Star Trek Scrapbook has the full interview) that the warp core itself had originally been a chrome tube, but this was changed at the insistence of Director Robert Wise. The blue light effect was created using lasers and strips of silver leaf on clothes hangers.
By turning them and hitting them with light, they created this tremendous effect of pulsating power, which gave off a bluish glow.
Given that main engineering is supposed to situated in the front of the stardrive section, the long corridor running north of it does not make. But this did not bother Michelson, who argued the important thing was to “open things up” and give the set a sense of scale.
Sources for this story include: David Houston, “Production Designer Harold Michelson, Visualizing the New Star Trek,” Starlog 30 (January 1980) 42-46; Steve Mitchell, “The Designing of Star Trek: The Motion Picture — An Exclusive Interview with Harold Michelson, Star Trek Art Director,” Fantastic Films 14 (February 1980) 8-15; and Star Trek: The Magazine 2, #8 (December 2001)