The project to upgrade the city’s 140-year-old suburban train lines will create an uninterrupted high-capacity commuter rail system of 76.3 kilometers between the suburbs of Gebze on the Asian side and Halkalı on the European side. It will include a line running through an immersed tube tunnel currently being built under the Bosphorus Strait. 

Of the five international consortia that submitted bids, a joint venture of San Francisco-based Bechtel and Turkish contractor Enka came in highest at $2.16 billion; followed by a joint venture of China’s CRCC and Turkish firms Cengiz Insaat and IC Ictas at $1.97 billion; a joint venture of Italian contractor Astaldi with Turkey’s Yüksel and Gülermak at  $1.62 billion; and a joint venture of Italy’s Impregilo and Ansaldo with Turkish firm Alarko, which bid $1.56 billion.

The Marmaray project includes an immersed-tube tunnel, bored tunnels, cut-and-cover tunnels, at-grade structures and construction of three new underground stations, as well as renovation and upgrade of 37 surface stations—many of them historic. An upgrade of existing tracks will include  a new third track on ground, completely new electrical and mechanical systems and procurement of modern railway vehicles.

While the suburban trains will go under the Bosphorus, a high-speed train now under construction between the capital of Ankara and Istanbul, the country’s largest city and commercial hub, will cross the strait on the third Bosphorus bridge now under way.

Habib Soluk, Turkey's Minister of Transportation, announced in April that the high-speed train will split into two lines when it reaches the outskirts of Istanbul, with one line connecting to the new bridge and the other linked to the Marmaray underwater rail crossing.

The tender for the $6.5-billion Northern Marmara highway project, which includes the third bridge, will be held in August. The project, totaling about 414 km of construction, is set to be build-operate-transfer. The new bridge is expected to stretch 1,275 meters across the Bosphorus.

Cahit Turhan, director general in the Transportation Ministry’s General Directorate of Highways, says that bidding companies will have operating rights for 20 years, “which we think is quite reasonable.” He expects project completion in five years. According to the ministry, 12 firms—six local and six from Russia, France, Italy, Spain and Austria—have officially indicated they will participate in the tender.