The Haydarpaşa–Gebze line is a commuter rail line operated by the Turkish State Railways, between Haydarpaşa Terminal in Kadıköy to Gebze (Turkey). Since its opening in 1969, it has been the busiest commuter rail line and busiest TCDD service in Turkey.

The Haydarpaşa–Gebze line was the second commuter rail service in Turkey, after the İstanbul–Halkalı line, operating across the Bosphorus. Although there had been steam service between Haydarpaşa and Gebze, it was in 1969 that the line was electrified and frequent commuter service began.

The route traverses the southern shore of Istanbul's Asian side, passing through important districts: Kadıköy, Maltepe, Kartal, Pendik and Tuzla. The line then crosses into the Kocaeli Province into Gebze, where it terminates. The State Railways has operated its regional service to Adapazarı and mainline service to Ankara on the line as well, making it the busiest rail line in Turkey, in passenger count and number of trains.

A railway to Gebze was originally built by the Ottoman Empire in 1873, as part of the railway to İzmit. This line was at first built for the Sultan, but later served the heavy population along the northeastern shores of the Sea of Marmara. However, few trains ran on the line during the time when the government operated it. The Ottoman government soon sold the line to a private enterprise, the Anatolian Railway (CFOA). The CFOA started regular train service between İstanbul and İzmit (as well as Gebze) in the 1890s. Service was suspended from 1914 to 1919 during the First World War. After the Republic of Turkey was formed in 1923, the CFOA continued to operate commuter service until 1927, when the Turkish State Railways acquired the CFOA. The newly formed State Railways (TCDD) continued the commuter service formerly provided by the CFOA.

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