Marmaray is a 76.6-kilometre-long (47.6 mi) commuter rail line located in Istanbul, Turkey. The line runs from Halkalı, on the European side, to Gebze, on the Asian side, along the north shore of the Sea of Marmara. Mostly using the right-of-way of two existing commuter rail lines, the Marmaray line linked the two lines via a tunnel under the Bosporus strait, becoming the first standard gauge rail connection between Europe and Asia. The two existing sections of the line were rebuilt and expanded from two-tracks to three-tracks, to allow for higher capacity with intercity and freight rail. The name Marmaray is a portmanteau of the words Marmara and Ray, which is Turkish for rail.

Gebze, formerly known as Libyssa, is a municipality and district of Kocaeli Province, Turkey. Its area is 418 km2, and its population is 407,019 (2022). It is situated 65 km (30 mi) southeast of Istanbul, on the Gulf of Izmit, the eastern arm of the Sea of Marmara. Gebze is the largest district by population size in the province as of 2022, exceeding İzmit, the provincial capital.