Kocaeli subway tunnel consists of twin tube 15,6 kilometre tunnels and 11 stations. Diameter is 6,5 metre. Now tunnel excavation continue 52 meters below ground. Completion of subway tunnels expected to finish in 2023. Importance of this tunnel is it connects Organized Industrial Zone's (OSB) in Kocaeli province are connected to Marmaray.

Mayor also said about another tunnel project. From Kocaeli to Sabiha Gökçen Airport in İstanbul. But it is still in early planning and negotiation phase with İstanbul province.

http://www.gebzegazetesi.com/gundem/gebze-darica-metrosunda-calismalar-devam-ediyor-h32061.html