Based on... What exactly? The Dutch rail network is already considered the busiest network in Europe. So I'm really interested what knowledge you bring that says that it could be busier if many experts say that the busiest trajectories (look at Amsterdam - Schiphol and Amsterdam - Utrecht, for example) are at max capacity already.
*) If you want to absolutely maximize passengers in a train you can reduce the number of toilets to one or switch to the peebag system the old trains had. And reduce the amount of wheelchair spots to the absolutely lawfully required amount. I’m not saying this is a good thing, just spinning some ideas to increase passenger capacity.
And if you include metro rail there are plenty of more frequent rail systems including again Japan but also China, France, UK, Hong Kong, Singapore, South Korea.