for its entire length (Fern Rock portal to Snyder), the tunnel was built with provision for 4 tracks only from the portal to just north of Lombard-South. At the time of opening, the outer 2 tracks were built along this length, whereas the inner 2 express tracks were built only in two sections, from the Fern Rock portal/shops to just south of Olney, and from Girard to their terminus just north of Lombard South. To close the gaps, the two inner express tracks were laid from Erie to Girard in 1959, and again from Olney to Erie in 1991. From Lombard-South station south to Snyder, the tunnel was constructed differently – only the eastern half of the line was built. The track currently used for southbound trains is actually the northbound express track. The extension in 1973 to Pattison station (now called NRG station) continued this arrangement. Space exists under the western half of Broad Street for the construction of the western half of the tunnel, which would include the remaining 2 tracks and additional island platforms for southbound local and express trains. The resulting infrastructure would match the configuration built in the northern half of the line. Provisions for flying junctions exist in the tunnels at three locations: north of Olney station, north of Erie station, and between Tasker-Morris and Snyder stations. These were to connect to planned but never built extensions to the north, northeast, northwest and southwest. Tracks were laid in the upper levels of the flying junctions north of Olney and Erie; these have been used over the years to store out-of-service trains and as layover points for express and Ridge Spur trains. The NRG Station contains a lower level platform (but very narrow compared to the very wide upper level platform), built to accommodate additional trains for large crowds at sporting events. Seldom used in recent years, these tracks are most often used to store rolling stock and work trains. Two of the Broad Street subway system's stations have been closed: Spring Garden station (closed in 1991) on the Ridge Avenue spur line and Franklin Square station on the PATCO route (closed in 1953 and later opened in 1976 for Bi-Centennial and then closed again in 1979). The Broad Street Line is one of only two rapid transit lines in the United States outside of New York City to use separate local and express tracks