South of Schiphol the dedicated high speed tracks begin, parallelling the existing railway line until Nieuw-Vennep. The line then branches off eastwards, continuing along the west side of Roelofarendsveen and Hoogmade and entering a tunnel east of Leiderdorp. This tunnel was built to protect the character of the Groene Hart region. North of Zoetermeer the train line leaves the tunnel west of Hazerswoude; it subsequently passes to the east of Benthuizen, and on an elevated track east of Zoetermeer, then back on the surface between Berkel en Rodenrijs and Bergschenhoek, and after a tunnel, joins the existing line again north of Rotterdam.