catwalk

noun

1. An elevated enclosed passage providing access fore and aft from the bridge of a merchant vessel.
2. Any similar elevated walkway.

   “The lengthening [of the District Line platforms] is being done so that passengers will no longer have to use the narrow catwalks to enter and leave the front and rear portions of eight-car trains.”

3. A narrow elevated stage on which models parade.
4. (broadly)The business of making clothes for fashion shows.