M4 (Istanbul Metro)
|Status||Opened (4 stations are under construction)|
|Stations||19 (4 stations are under construction)|
|Daily ridership||134,524 (2013)|
|Opened||17 August 2012|
|Owner||İstanbul Metropolitan Municipality|
|Operator(s)||İstanbul Ulaşım A.Ş.|
|Rolling stock||144 CAF|
|Line length||26.5 km (16.5 mi)|
|Number of tracks||2|
|Track gauge||1,435 mm (4 ft 8 1⁄2 in) standard gauge|
|Electrification||1,500 V DC Overhead line|
|Operating speed||100 km/h (62 mph)|
M4, officially referred to as the M4 Kadıköy–Tavşantepe line (Turkish: M4 Kadıköy–Tavşantepe Metro Hattı), is a 26.5-kilometre (16.5 mi), 19-station rapid transit line of the Istanbul Metro. Running between Kadıköy and Tavsantepe, it is currently the only rapid transit line operating on the Asian side of Istanbul. The M4 mostly runs under State road D100, parallel to the Istanbul-Ankara Main Line and is entirely underground.
The M4 opened on 17 August 2012 with a large ceremony in Kadıköy, in which Recep Tayyip Erdoğan personally took part in. On 29 October 2013, with the opening of Marmaray rail service under the Bosphorus, Ayrılık Çeşmesi station was opened to allow passengers to transfer between lines.On 10 October 2016, 3 new station added to the line (Yakacık Station, Pendik Station and Tavşantepe Station). 
A plan to build a subway line on the Asian side of İstanbul was approved in 2005. In 2008, a €751 million construction contract was awarded to a consortium of Astaldi, Makyol and Gülermak. The rolling stock was ordered from the Spanish company CAF. In mid-2010, the construction of the tunnels was completed and the track laying started. The route has connections with TCDD's Marmaray system at Ayrılıkçeşmesi and the İstanbul Metrobus system at Ünalan. Scheduled opening date was December 2011 up to Kartal, however it was postponed due to signalization works.
- Kadıköy (transfer to: T3, İDO)
- Ayrılık Çeşmesi (transfer to: Marmaray)
- Ünalan (transfer to: Metrobüs)
- Kozyatağı (future transfer to: M8)
- Hastane - Adliye
- Hastane (under construction)
- Şeyhli (under construction)
- Kurtköy (under construction)
- Sabiha Gökçen International Airport (under construction)
The stations Kadıköy, Ayrılıkçeşmesi, Ünalan and Göztepe are situated below sea level. The average distance between the stations is 1,300 m (1,400 yd). The distance between the stations at Maltepe and Huzurevi is the shortest at 800 m (870 yd). The distance between Bostancı and Küçükyalı is the longest with 2,300 m (2,500 yd). The Operation Control Center of the line is situated in Esenkent and the depot is located in Maltepe. Another depot is to be built in Kaynarca in 2016 when the line extends there .
Train frequency is currently every 150 seconds during peak hours and could be shortened to 90 seconds later on. Trains remain at stations for an average of 15 seconds. Four-car trains have a 1,000 passenger capacity and eight-car trains have 2,000 capacity. Ridership capacity in one direction is 70,000 passengers hourly. Total travel time of the entire line is around 40 minutes.
M4 is operated by CAF trains (144 cars in total) every 150 seconds during peak hours. The frequency is much less on weekends and during off-peak hours.
- "Kadıköy-Kartal Metro Hattı 144 Adet Metro Aracı Alımı İşi" (in Turkish). İstanbul Büyükşehir Belediyesi. Retrieved 2013-11-16.
- "M4 Kadıköy-Kartal Metro Hattı" (in Turkish). İstanbul Ulaşım. Retrieved 2015-10-04.
- "Yüzde 50 müjdesiyle geliyor". Hürriyet (in Turkish). 2012-08-17. Retrieved 2013-11-16.
- "Kadıköy - Kartal Metrosu Ayrılıkçeşme İstasyonu ihalesi" (in Turkish). Emlak Kulisi. 2012-07-11. Retrieved 2013-11-16.
- "Marmaray tunnel opens to link Europe with Asia". Railway Gazette International. 29 October 2013. Retrieved 4 October 2015.
- "İstanbul Büyükşehir Belediyesi". www.ibb.istanbul. Retrieved 2016-10-10.
- "Kadiköy - Kartal metro opened by Prime Minister". Railway Gazette International. 17 August 2012. Retrieved 4 October 2015.
- "Kadıköy-Kartal Metro Hattı Temmuzda Açılacak" (in Turkish). İstanbul Net. 2012-03-02. Retrieved 2013-11-16.
- "Kadıköy-Kartal Metro Hattı" (in Turkish). İBB. Retrieved 2012-12-11.
Media related to Istanbul Metro line M4 at Wikimedia Commons