Upgrading to full ERTMS can increase capacity on the Utrecht Amsterdam line. Longer platforms with longer trains. Reducing accessibility in the trains* for more seating. Less seats. Removing first class. Remove train crossing. Buy out NIMBIES. There is plenty you can do to increase passenger capacity on that line.

*) 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.