Work on the Manhattan side included building a new 8-track train station with storage tracks extending to 38th Street. North of the station, the tunnels would connect to the 63rd Street Tunnel's lower level. The new station would be housed in two gigantic caverns blasted out of the Manhattan schist rock formation under the station. A three-level structure was being built in each cavern, with one train platform and two tracks each on the top and bottom levels. A passenger concourse was being constructed between each cavern's upper and lower levels. An upper-level concourse, accessed by a series of stairs, elevators, and escalators, would replace ten tracks on the west side of Metro-North's Madison Yard. The cavern structure is located under the Park Avenue Viaduct, which surrounds Grand Central Terminal, while the storage tracks are located under the Park Avenue Tunnel, which is located south of Grand Central Terminal.