Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 .
Simplified: 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 .