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 . It is the only line on the Underground , except for the two-station Waterloo & City line , that is operated entirely underground , the only section of track to emerge above ground being the part from Seven Sisters to the line 's depot at Northumberland Park .