When service of this line started , the section of the Kwun Tong Line from Chater to Argyle , present-day Mong Kok station , was transferred to the Tsuen Wan Line . When service of this line started , the section of the Kwun Tong Line from Chater to Argyle ( since renamed Central and Mong Kok stations respectively ) , was transferred to the Tsuen Wan Line .