On the line you mention its no more-only SPRinter=all stops trains shuttling AMs Central to Schiphol Airport, 6 or 8 per hour. Major reason is in fact the rebuilding of AMS Central to get wider platforms-so all tracks have to be realigned- to be able to get space for the masses.
On where I live and now send from: between den Haag and Rotterdam, still sometimes called ´the old line´, NS has partly and is busy putting on ETMET= Every Ten MInutes Een (=a) Trein/train, which applies to both the all stops, which I have to take to get home and the IC=faster trains. Its 4 tracks going south from den Haag HS till just beyond Rijswijk (tunnel station), then still 2 but planned to be also 4 tracks till just before tunnel of new station Delft (technical university, once so famous for waterworks engineering) then again 2 between the meadows with the odd windmill till Schiedam, where it meets the now metro line from Hook of Holland (where the ferry from UK arrives) and onward. This line is -I use it 3/4 times/week, quite punctual and reliable and so they do manage to get even more trains/hr on that section south of Delft. But why even ask? ANY metroline can also do that and most regulars very well know that NS is more like a big-size metro (underground for UK, though it only has a few tunnels here and there) running trains. And I always kind of thought that your old SR=Southern Region also has some pretty frequent lines with a similar service on it?
In fact lines with heavy freight are those most prone to heavy delays, as I also again experienced yesterday.
But again reading more thorough your post: On THAT section it is rail from Airport=Schiphol beside metro/GVB from Zuid=South. These lines are not linked.
At that time the train=NS line did have at least 4 SPRinter=all stops and 4 IC-s-a few more at some specific times.
