M4 is a rapid transit

Rapid transit

A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

 line of the İstanbul Metro

Istanbul Metro

The Istanbul Metro, or the M2, is a mass-transit underground railway network that serves the city of Istanbul, Turkey. Founded in 2000, it now includes 13 stations. The system currently consists of a single north-south line...

, currently under construction in İstanbul

Istanbul

Istanbul , historically known as Byzantium and Constantinople , is the largest city of Turkey. Istanbul metropolitan province had 13.26 million people living in it as of December, 2010, which is 18% of Turkey's population and the 3rd largest metropolitan area in Europe after London and...

, with opening planned to be on December 31, 2011. The M4 will be the only rapid transit line on the Asian side of İstanbul. The line will start at Kadıköy

Kadiköy

Kadıköy is a large, populous, and cosmopolitan district of İstanbul, Turkey on the Asian side of the Sea of Marmara, facing the historic city centre on the European side of the Bosporus...

, on the Bosphorus shore near Haydarpaşa Terminal

Haydarpasa Terminal

The Haydarpaşa Terminal or Haydarpaşa Station is a major intercity rail station and transportation hub in Kadıköy, İstanbul. It is the busiest rail terminal in Turkey and the Middle East and one of the busiest in Eastern Europe. The terminal also has connections to İETT bus and ferry service. The...

, continue north to parallel with the Haydarpaşa-Gebze commuter line until Kaynarca

Kaynarca

Kaynarca, formerly Yena, is a town and district of Sakarya Province in the Marmara region of Turkey. The mayor is Zeynur Özel ....

 under E-5 highway. As of 2011, the Kadıköy-Kartal section is complete.

History

A plan to build a subway line on the Asian side of İstanbul was approved in 2005. Construction started shortly after. In mid-2010, the tunnel building was complete and the track laying started. The route with have connections with TCDD's Marmaray system at Ayrılıkçeşme and the İstanbul Metrobus system at Uzunçayır. Scheduled opening date is February 2011 up to Kartal.