numbers increased rapidly. Since the opening of the S-Bahn, travel volumes have increased by about 60%. In several stages, S-Bahn services were expanded to a rail network, and peak hour express trains were added. The first stage of the expansion addressed the chronic overcrowding of trains on the S12 route between Dietikon and Zürich, requiring improvements in the Limmat valley. The widening of the railway to four tracks between Dietikon and Killwangen allowed the separation of the S-Bahn from the long-distance and freight services. The new S3 service introduced as a result complemented the S12 service, to provide a train every 15 minutes on the route. At the same time the sections of the S9 route via Knonau were upgraded to allow services to be increased to each half hour. Services on the north side of Lake Zürich were increased with trains provided every 15 minutes by S6, S7 and S16 services. A new station was opened at Glanzberg between Dietikon and Schlieren. Under the second stage of expansion in December 2002, night trains were added to the S-Bahn. Since 2007, night trains provide a continuous 24-hour service from Friday morning until Sunday evening on some lines. The third stage expansion was completed in 2007. On 12 December 2004 (coinciding with the completion of the first stage of Rail 2000), the S3 service was extended from Dietikon through the Heitersberg Tunnel to Aarau with a new station at Mellingen. On 10 December 2006, S15 was opened between Rapperswil and Birmensdorf following line improvements. It was extended from Birmensdorf to Affoltern am Albis on 9 December 2007. South of Zürich sections of the Sihltalbahn were doubled. On the rural feeder lines around Winterthur (S33 to Schaffhausen, S35 to Wil and S41 to Bülach) services were increased to run every half hour. The S8 was extended from Winterthur to Weinfelden, providing with the existing S30 services two trains an hour on the line. In addition, the S16 was extended every hour to Schaffhausen, stopping after Winterthur only at Andelfingen and Neuhausen am Rheinfall. Moreover, on the line from Winterthur to Wil a new station opened at Winterthur Hegi. Opening of the Weinberg Tunnel Following a successful referendum, a project was established to create a new route between the Hauptbahnhof and Oerlikon station. Unlike the existing two routes between the stations, the Weinberg tunnel would approach the Hauptbahnhof from the east, allowing trains