Catwalk

From Memory Alpha, the free Star Trek reference.

You might also be looking for the The Catwalk, a Star Trek: Enterprise episode.

The nacelle catwalk in an NX class starship

A shematic showing the location of an NX class starship catwalk (highlighted)

The catwalk is the nickname both for a maintenance shaft inside a warp nacelle and for a maintenance walkway on Deck 35 just above the center of the warp core in Main engineering onboard Galaxy-class starships.

NX class starship catwalks were protected by an osmium alloy.

In 2152 the crew of Enterprise (NX-01) had to take refuge for eight days inside the starboard catwalk when a deadly neutronic storm threatened the ship. In preparation for this, Commander Tucker built a makeshift secondary command center in one of the nacelles by hastily rerouting several control systems. Later that year, many took refuge there again, this time to protect themselves from "Wisp" beings that would take-over the crews bodies. (ENT: "The Catwalk", "The Crossing")

In 2368 Lieutenant Commander Geordi La Forge was feeling dizzy and almost fell off the catwalk. He was rescued by Ensign Fletcher. (TNG: "Cause and Effect")