With a ticket on an IC train it is my understanding that you can indeed just hop on and off en route as you like - and you must change trains twice - once in Rotterdam where the train station is in the heart of the city - a fascinating and totally unique, for Europe, city.

I agree re Rotterdam. The city has been completely destroyed during the war and has been rebuilt with, at the time, the most modern buildings. The cube houses, for example, and another building that is known as Het Potlood (the pencil). In Rotterdam you can see a unique mixture of old and new.