Main engineering aboard the Galaxy-class USS Enterprise-D was equipped with at least two isolation doors. The outer isolation door served to separate the warp core and engineer's office from the rest of engineering, the inner door only separated the warp core from the rest of engineering, in an emergency, the engineer's office would still be accessible. In 2366, the outer door was brought down after a Borg attack caused a hull breach in engineering. (TNG: "The Best of Both Worlds" )