The K-stock was built jointly by Mitsubishi Heavy Industries and Hyundai Rotem and first put into service on the Kwun Tong line. Subsequently, in 2006, four additional sets joined the Tung Chung line to cope with the increasing passenger traffic. K-Stock trains have come under criticism when it was first put into service due to delays and door safety issues. There have been incidents where passengers have been injured by its doors and other service reliability issues have led to MTRCL "minimising the number of Korean trains for passenger service until a higher reliability of the systems concerned is achieved".