The construction of a railway tunnel beneath the Bosporus strait dates back to 1860, when SultanAbdulmejid I first proposed an undersea crossing of the strait. The project was proposed again in 1892, under Abdul Hamid II, when French engineers planned such a tunnel. The plan was never realized. Plans to build a modern trans-Bosporus tunnel were proposed in 1997, based on a feasibility study ten years prior, and the necessary capital secured in 1999. Preparatory works on the project began in 2001. Construction began in May 2004. The tunnel was constructed by a Turkish-Japanese consortium led by the Taisei Corporation. Tunnel construction was completed on 20 October 2008, and rails were added in 2009. The completion of the project was delayed due to archeological discoveries near Sirkeci, with artifacts dating back 8,000 years. Test runs began on 6 August 2013, and operations began 29 October, with a ceremony in Üsküdar.
Operations
The Turkish State Railways own the tunnel, while trains are operated byTCDD Taşımacılık. Both tunnels are electrified with 25 kV AC 50 Hzoverhead wire. The western portal to the tunnel is in Kazlıçeşme, Zeytinburnu on the European side of the city, just west of Fatih. Out of the three tracks that approach the west portal, two of them enter the tunnel, while the third track continues to Sirkeci station. The eastern portal to the tunnel is located in Ayrılıkçeşmesi, Kadıköy on the Asian side of the city. Ayrılık Çeşmesi station is located right outside the east portal. After Ayrılık Çeşmesi station, the two tracks connect to the Istanbul-Ankara railway, from Haydarpaşa station, and continue towards Gebze and Anatolia. As of November 2017, TCDD Taşımacılık operates 164 round-trips between Kazlıçeşme and Ayrılık Çeşmesi at intervals from every 5 minutes during peak hours, to 10/15 minutes during off-peak hours. In total, the Marmaray tunnels see 328 scheduled trains daily.
Engineering
The undersea section of the Marmaray tunnel is the deepest immersed tube tunnel in the world, with its deepest point being below sea level. This section of the tunnel is long and consists of 11 sections lowered via barges on the Bosporus. Eight of these sections have a length of, two of them have a length of and one with a length of. These immersed tube tunnels are connected via bored-tunnels on both sides. Before the tunnel sections were lowered, a portion of the ground underneath the Bosporus needed to be strengthened. This was done by lowering concrete columns 4 to 10 meters long which stabilized the area. The tunnels were assembled on dry-docks in Tuzla and tested off Büyükada. After testing, they were dragged by Tugboats to their positions at the southern end of the Bosporus. Once the tunnels were lowered into place, they were covered with fill, de-watered and sealed. Tunnel boring machines were used to drill the tunnels under land. The under land section of the tunnel consists of two tube tunnels, each with a single track. Walkways are present within each tube.
