The Istanbul suburban railway, locally referred to as B1 (Turkish: İstanbul Banliyösü), was an important rail line in İstanbul, Turkey. It was operated by the Turkish State Railways and was one of Istanbul's two commuter rail lines (the other being the Haydarpaşa suburban). Carrying an average of 22,200 passengers daily, it was the second-busiest commuter railway in Turkey, after the Haydarpaşa suburban.

The İstanbul Commuter Railway is a historical line, being the first rail line to be electrified and being the first commuter railway in Turkey. Since it opened on December 4, 1955, the E8000 series have been in operation. In the 1970s, the E14000 series came into service and in 2010 the new EUROTEM built E23000 series began service. The line was closed in 2014 and all tracks have been removed for renovation and incorporation into the new Marmaray network which started operation in 2013 between Ayrılıkçeşmesi and Kazlıçeşme.

The eastern terminus is Sirkeci Terminal in the Fatih district in the historical city center. The line curves around the shore of the Eminönü peninsula, below the historic Topkapı Palace, and heads west. Travelling on the southern European shore of Istanbul, the line passes through several important districts, until curving north at Soğuksu to Halkalı.

The line was originally built by the Oriental Railway (CO) in 1872, as a part of their İstanbul–Vienna main line. The railway started to operate trains, when the line reached Edirne in 1873. The CO was absorbed by the Turkish State Railways (TCDD) in 1937 and TCDD started to operate steam-powered commuter service on the line. It wasn't until 1955, when the line was electrified from Sirkeci to Halkalı, that a frequent commuter service began. The newly retired E8000 EMUs were the first to run on the line along with three E4000 electric locomotives. In the 1970s, new E14000 EMUs started to appear on the line and in the 1990s, the E4000 locomotives were retired. New E23000 EMUs were put into service on September 19, 2010. With the addition of the new EMUs, the E8000s were finally retired.

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