M4 Kadıköy - Sabiha Gökçen Airport Metro Line

The M4 Kadıköy - Sabiha Gökçen Airport Metro Line is one of the rail systems serving the Anatolian side of Istanbul. Its primary purpose is to begin at Kadıköy, run parallel to the Marmara Sea coastline, and follow the D100 highway route to connect the densely populated areas of the Anatolian side with Sabiha Gökçen International Airport.

The tender process for the M4 Metro Line began on 14 January 2008, with the contract signed on 6 March 2008 and construction commencing on 21 March 2008. Key milestones included the delivery of the first train on 11 January 2011, its placement on the rails on 27 January 2011, completion of the tunnels in October 2011, and activation of the signaling system in March 2012. Trial runs commenced on 8 May 2012. The first and foundational phase of the line, between Kadıköy and Kartal, opened for service on 17 August 2012. At this stage, the line operated over a 21.7 km length with 16 stations. The opening of Ayrılık Çeşmesi Station on 29 October 2013 enabled integration with Marmaray. The second extension phase, between Kartal and Tavşantepe, opened on 10 October 2016. Finally, the third phase, extending from Tavşantepe to Sabiha Gökçen Airport, opened on 2 October 2022, providing direct access to the airport. With the completion of this final phase, the total length of the line reached 33.5 km and the number of stations increased to 23.^【1】 

The M4 Line begins at Kadıköy and follows the alignment of the D100 highway through the Acıbadem area, continuing beneath this major artery all the way to Tavşantepe. Beyond Tavşantepe, it passes through residential areas north of the D100 to reach Sabiha Gökçen Airport. During construction, Tunnel Boring Machines (TBMs) were used between Kadıköy and Kozyatağı and between Kartal and Kaynarca, while the New Austrian Tunnelling Method (NATM) was employed between Kozyatağı and Kartal. The entire line was constructed underground, with station platforms designed to be 180 meters in length. To support operational infrastructure, a depot with a capacity of 52 vehicles (13 train sets) and a maintenance workshop with a capacity of 32 vehicles were built between Maltepe and Huzurevi stations, and in the vicinity of Maltepe Station.^【2】 

The M4 Line consists of the following 23 stations, running from Kadıköy to Sabiha Gökçen Airport:

^{Stations on the Sabiha Gökçen Airport - Kadıköy Metro Line (}Metro İstanbul⁾

The line has a total length of 33.5 km and serves 23 stations. A total of 144 vehicles operate in 36 four-car train sets. The one-way journey time is 52 minutes. The M4 Metro Line operates daily from 06:00 to 00:00. During peak hours, service frequency drops to every 5 minutes, while off-peak intervals extend to every 9 minutes. On certain days, particularly Friday and Saturday nights, additional services run every 30 minutes from 00:30 to 05:30; during these night services, a double-fare tariff applies. The line is reported to have a daily passenger capacity of 300,000 and operates 386 trips in one direction per day.^【3】  Hattın günlük yolcu taşıma kapasitesinin 300.000 kişi olduğu ve tek yönde günlük 386 sefer düzenlendiği belirtilmiştir.^【4】 

The M4 Metro Line employs advanced technologies to maintain high levels of passenger safety and system reliability. The signaling system for the line and depot is based on Thales CBTC (Communication-Based Train Control), operating on a continuous communication-based moving block principle. Security is monitored continuously through a network of 1,281 cameras installed at stations, with additional checks conducted by security personnel. Emergency scenarios for smoke management and passenger evacuation have been developed and tested via simulations. Materials used in tunnel construction and equipment selection avoid the emission of toxic gases and withstand high temperatures in case of fire. A reliable, fully automatic smoke control and evacuation system compliant with NFPA standards ensures rapid and safe passenger egress during fire emergencies. Power supply is provided from three separate points to ensure system continuity. In the event all three supply points fail, generators located at both ends of the line activate to move trains to the nearest station and enable passenger evacuation. In the rare event that the generators also fail, lighting and electronic control systems can operate uninterrupted for up to three hours via uninterruptible power supplies (UPS).

The M4 Metro Line plays a central role in Istanbul’s transportation network by integrating with multiple other public transit modes at numerous points. Key transfer points and connections include:

• Kadıköy Station: Provides connections to the T3 Kadıköy - Moda Tramway Line, city buses, sea buses, and motorboats.
• Ayrılık Çeşmesi Station: Offers direct interchange with the Marmaray system.
• Ünalan Station: Provides transfer access to the Metrobüs system.
• Kozyatağı Station: Connects to the M8 Bostancı-Parseller metro line.
• Pendik Station: Offers transfer to Pendik High-Speed Rail Station (YHT) via İETT buses.