Simplify the following text for a Grade 8 native English-speaking person.
Original: The MRT system had relied on its two main lines , namely the North South Line and East West Line , for more than a decade until the opening of the North East Line in 2003 .
Simplified: The MRT system had been using its two main lines , the North South and East West Lines , for more than ten years until the opening of the North East Line in 2003 .