Marmaray is a 76.6 km (47.6 mi) long commuter rail line in Istanbul, Turkey. It comprises a rail tunnel under the Bosphorus strait, and the modernization of existing suburban railway lines along the Sea of Marmara from Halkalı on the European side to Gebze on the Asian side. The procurement of new rolling stock for suburban passenger traffic is also part of the project. Construction started in 2004, with an initial target opening date of April 2009. After multiple delays caused by the discovery of historical and archaeological finds, the first phase of the project opened on October 29, 2013. It is the first standard gauge rail connection between Europe and Asia. The second phase of the project was scheduled to open in 2015, but the work was stopped in 2014. The work was restarted in February 2017 and the full line opened on 12 March 2019.
Marmaray is the fourth underground system in Istanbul after Tünel Funicular, Metro and Kabataş–Taksim funicular and highly integrated with Metro lines.
The name Marmaray comes from combining the name of the Sea of Marmara, which lies just south of the project site, with ray, the Turkish word for rail. The Turkish press has compared it to the Silk Road.