The other lines are operated using a mixture of the other two variations, known as the "M-Train" and the "K-Stock". The "M-Stock" (or CM-Stock") of "M-Train" are the oldest trains on the MTR, built originally by Metro Cammell (now Alstom) and refurbished by United Goninan. The "M-Train" is the only variation that uses sliding doors, as opposed to others which use plug-doors. The "K-Stock" are built jointly by Mitsubishi Heavy Industries and ROTEM, and a further order of 32 cars is due to be in service on the Tung Chung Line by 2007.