The cramped maintenance shaft running the length of each warp nacelle on Enterprise NX-01, where the whole crew sheltered for eight days during a neutronic storm in 2152.
The catwalk is a narrow maintenance passage inside each of Enterprise's warp nacelles. It gives its name to the second-season episode The Catwalk.
When a huge and fast-moving neutronic wavefront overtakes the ship in 2152, its radiation would kill everyone on board. The only shielded space big enough for the crew is the catwalk, which is lined with a dense alloy. With the warp engines shut down so that the nacelle does not become lethally hot, the entire crew, Porthos and three alien visitors squeeze in for the eight days it takes to cross the storm.
The visitors prove to be deserters from the Takret Militia, whose former comrades board the apparently empty ship. Archer, T'Pol and Reed go out in environmental suits to drive them off.