(covers information from several alternate timelines)
Engineering, or main engineering (called reactor pit in Klingonese), is the location from which the ship's main power systems are controlled. Engineering is the primary assignment of engineers and chief engineers.
In the late 20th century, nuclear vessels of the United States Navy designated their engine room as the main machine room, or m.m.r. A ship, like the aircraft carrier U.S.S. Enterprise, had at least four main machine rooms. Each of these rooms had a nuclear fission reactor. (Star Trek IV: The Voyage Home)
Engineering's primary purpose is to be the central point for control of all engineering systems aboard a starship, especially those related to propulsion and power generation. The matter/antimatter reaction chamber (also known as the warp core) is located in engineering.
22nd century design
NX-class starships of the 22nd century were the first Human vehicles to carry a warp reactor capable of speeds up to warp 5. (ENT: "Broken Bow") Aboard an NX-class ship, a section of engineering was situated on E Deck. (ENT: "Zero Hour")
23rd century design
On the original Constitution-class starships, engineering was a two-deck-tall room that housed the main dilithium reactor and the engineering core as well as a large transparent grill that overlooked power conduits. The room was extensively refitted on Constitution-class starships in the late 2260s to include additional computers, offices, and rooms around the main area. (TOS: "The Corbomite Maneuver", "The Naked Time", "The Conscience of the King", "The Ultimate Computer", "Mirror, Mirror", "Day of the Dove")
Towards the end of the 23rd century, the engineering department on refit Constitution-class starships was drastically changed, spanning multiple decks. The uppermost level served as the structural support strong back of the ship, and was the anchoring framework for the connecting dorsal and the warp nacelle pylons.
On the forward end of this level was the engineering computer monitoring room, which encircled the intermix shaft and opened, to the rear, into the engineering computer bay. The rear bulkhead of the computer bay contained an emergency section door which lowered to the deck below, and separated the warp engine room from the extended horizontal intermix area; the door dropped automatically in the event of a radiation leak or pressure loss.
On the Constitution-class ship, a narrow corridor bypassed the computer bay on the port side and led aft, down the center of the level. On either side of this passageway were mounted the four maneuvering thrusters which rested beneath the upper hull of the secondary hull strong back.
These thrusters were used for vessel course control when within close proximity of drydock facilities.
The lower engineering deck typically housed the engineering department. Located in the center of the room, and extending for many levels both above and below the deck, was the vertical linear intermix chamber.
This complex, a radically new design in intermix technology, provided operational power for the impulse drive system and furnished enough additional energy to power all other shipboard systems. Both matter and antimatter for this chamber were contained in a series of magnetic bottles, which were normally housed in pods at the base of the intermix shaft.
These pods could be ejected from the ship in case of an extreme emergency via two large blow-away panels in the outer hull. (Star Trek: The Motion Picture)
24th century design
24th century starships featured a more modern approach to the engineering facility. Aboard the Galaxy-class starship, engineering was an open-plan facility, directly accessible from the corridor. Consisting of two primary levels, it housed the starship's warp core and primary engineering support systems.
The corridor bulkhead housed the master systems display. Inside the main section, the master systems display was the operational focus of the room.
Beyond this, heading towards the warp core, the chief engineer's office and several support consoles were located on the left, and the assistant chief engineer's console on the right. These formed part of the bulkhead protecting the main part of engineering from the warp core.
- Greg Tyler. "Where on the USS Enterprise is the Engine Room?", 1 January 2006, at Trekplace.com (30 November 2006)