For decades, the marshalling yard has been a barrier for pedestrians wanting to get to the Waalhaven port. This bridge now finally nullifies the barrier, while creating a safer location in the process, as before people used to cross the tracks on foot.
The presence of the marshalling yard is key in the slender design. Because trains are only allowed to travel at very low speed, there is basically no risk of collisions. Therefore, the bridge does not have to be able to withstand collision loads, which has resulted in a very slender bridge structure.
Furthermore, the choice of structure also contributes to the slenderness.