M11 (Istanbul Metro)

Line M11, officially referred to as the M11 Gayrettepe–Istanbul Airport–Halkalı Metro Line (Gayrettepe–İstanbul Havalimanı–Halkalı Metro Hattı), is a line of the Istanbul Metro. , it is operational between Gayrettepe and Arnavutköy Hastane via Istanbul Airport. The line is currently 51.5 km long, making it the longest line in the Istanbul Metro. The M11 is entirely underground, making it the second longest continuous subway tunnel in Europe, behind the Moscow Metro's Bolshaya Koltsevaya line.

Route
The purpose of this line is to provide direct and quick access from Istanbul Airport in Arnavutköy to Levent, Istanbul's main commercial center and to Halkalı, one of the city's most important train stations. Unlike other metro lines, the M11 will be a limited stop service, meaning that stations will be spaced further apart. This is most noticeable towards the line's southern ends, where the M11 will only have seven stations within Istanbul's urban area: Halkalı, Halkalı Stadı, Olimpiyat, Kayaşehir and Fenertepe in the west and Gayrettepe and Kâğıthane in the east.

It will have two legs, one from Gayrettepe to Istanbul Airport line and the second from Halkalı to Istanbul Airport line. The first leg of the rapid transit line will be 37.5 km long with nine stations. The second leg of the line will be 31.5 km long with seven stations, in a total of 69 km with sixteen stations. When fully completed, the M11 will be one of the longest continuous subway tunnels in the world. The travel time between the end stations will be 60 minutes at a maximum speed of 120 km/h. The line will run through six districts of Istanbul, Şişli, Kağıthane, Eyüpsultan, Arnavutköy, Başakşehir and Küçükçekmece. It is expected that the line will carry annually around 94 million passengers. The M11 line will connect to other rapid transit lines: Marmaray, M1, M2, M3, M7 (twice), M9  and at the airport to the high-speed train line.

Construction
The investment budget of the construction is €999.8 million. The Turkish construction consortium Kolin-Şenbay won the tender for the building of the metro line in December 2016. The line was not scheduled to be completed before the airport's official October 29, 2018 airport opening. The line is being constructed in four sections:
 * Kağıthane–Istanbul Airport, opened on 22 January 2023
 * Gayrettepe–Kağıthane, opened on 29 January 2024
 * Arnavutköy Hastane–Istanbul Airport, opened on 19 March 2024
 * Halkalı–Arnavutköy Hastane, predicted to open in early 2025

Rolling stock
The rolling stock was manufactured by CRRC Zhuzhou Locomotive, the first 120 km/h class automated metro train exported by China. 176 metro cars were supplied for the Line 11. The contract was awarded in January 2020, and the delivery was completed six months later, although these ten sets were not automated. On January 3, 2024 the Minister of Transport and Infrastructure, Abdulkadir Uraloğlu announced that 15 automated trainsets were ready to enter into use.

Stations
The M11 has sixteen stations. Out of these sixteen, only eight are within the Istanbul metropolitan area.