I got it from here:
https://en.wikipedia.org/wiki/Passengers_per_hour_per_direct...Heavy rail metro refers to a system like the NYC subway, whereas Caltrain would be classified as suburban rail.
A typical NYC subway train has 10 cars with room for 100 passengers each, so to achieve 60k p/h/d would require 60 second headways. This is shorter than the 90 second headways achieved in practice, though theoretically possible with automatic train control and platform screen doors. For a more realistic example, the cars have a crush loading of around 150 passengers, which means 60k p/h/d is possible with 90 second headways.
Suburban rail actually has a higher theoretical capacity than heavy rail metro, but this is achieved with very large trains rather than high frequency. Caltrain's new Stadler KISS EMUs will have a capacity of around 200 passengers and will operate in 7 car trains and 10 minute headways, so they will only achieve 8.4k p/h/d initially. However if we assume 300 passenger crush loading and 10 car trains, it could theoretically achieve 60k p/h/d with more reasonable 180 second headways.
Now running trains at crush load routinely doesn't make for a great passenger experience, but keep in mind that we are assuming a dual track railroad here. If you are willing to spare the extra 2 highway lanes' worth of right of way, you can lay quad track and run twice as many trains. Many NYC subway lines were built quad tracked so that express trains could pass locals, and Caltrain's ROW is more than wide enough to accommodate retrofitting another pair of tracks.
