Original: It is the only line on the Underground, except for the two-station Waterloo & City line, which is operated entirely underground, with the only section of track to emerge above ground being the route to the depot from Seven Sisters to Northumberland Park.
Simplified: It is the only line on the Underground, other than the two-stop Waterloo & City line, which is completely underground. The only section of track to come above ground is the route to the depot from Seven Sisters to Northumberland Park.