For example, on practically the entire route, the rails are laid on pile supported no-recess concrete slab. This type of railway is also known as ballast-free rails or Rheda rails, and is unique in the Netherlands. Especially for the HSL, the contractor developed several machines to lay the Rheda rails. For the construction of the bored tunnel under the Green Heart Area , the largest tunnel-boring machine in the world was built. But the project also employs traditional methods and techniques, such as underwater concrete.