The hard limit is a function of stops, layout, train length and acceleration/deceleration ability, and finally the signal system.

Each train has to approach a station, brake, stop, wait while people enter and leave, start and accelerate, all at safe distance to the trains in front and behind. Braking, standing still and starting easily add up to a minute, so a two-minute cycle time is hard even if the track layout is friendly (evenly spaced stations, no mutexes in the central area, etc).

Ninety seconds is really impressive.

40tph is rarely achieved in practice, systems which are intended for 40tph usually actually end up running somewhere in the 30-36tph range as it basically just takes one little old lady being a bit slow or one tourist bag stuck in a door to ruin the synchronisation. Still 36tph basically already feels like there are trains "all the time" and passengers learn to just "let it go" as you would with an elevator - you know another one will arrive shorty.

Getting rid of absolute block helps, a moving block system can allow the following train to enter immediately, rather than waiting until the tail of train A leaves the far end of the station before the front of train B can begin moving. But that's not exactly something you can just drop in on a Tuesday afternoon, you'd usually have to shut the whole railway, add new signal technology, re-train staff, maybe even buy new rolling stock.

Are there others that try to operate >30tph? Which ones?

I've only travelled with one 30tph (two-minute) system, it was really relaxing. As you say, it felt as if the next train was coming immediately. Quite subjectively, it felt as if the wind pushed by next train could be felt while the previous train's lights were still visible in the tunnel.

The Victoria Line in London runs 36 tph. The Jubilee Line is 30, but will be 36 by 2019. The Central Line has 35tph in the morning.

https://en.wikipedia.org/wiki/Victoria_line#Service_and_roll...

https://en.wikipedia.org/wiki/Jubilee_line#Service_frequency

https://en.wikipedia.org/wiki/Central_line_(London_Undergrou...

No subway system in the world runs 40tph with western safety standards; the Moscow Metro achieves that rate because they leave it to the passenger to clear the doors before the train departs.

St. Petersburg as well. (Actually, closer to one per 2 minutes.)