40 trains per hour is in fact not "normal", but extremely difficult. Only a few systems in the entire world operate more than 30 per hour.
The fundamental constraint is not technology, but people and physics: you need to decelerate and stop, let people disembark and get on, accelerate and clear the platform. This cycle requires a bare minimum of 90 seconds, although IIRC a few lines in a few places like Paris and Moscow do 85 secs.
Indeed, the Victoria line in London manages 36 TPH and we've not bothered beating it since. It's much easier to run 26-30TPH with slightly more carriages.
> the Victoria line in London manages 36 TPH and we've not bothered beating it since
That was a world record for a line following modern safety standards, set less than 10 years ago. It's hardly a case of "not bothered", it's just hard.
90 seconds is very possible in new-build lines which is what the author is talking about. You can buy a turnkey Innovia (e.g. Vancouver Skytrain) or AnsaldoBreda (e.g Copenhagen) that does this out of the box. Retrofitting 90s operation is basically impossible but not the point of this exercise.
The fundamental constraint is not technology, but people and physics: you need to decelerate and stop, let people disembark and get on, accelerate and clear the platform. This cycle requires a bare minimum of 90 seconds, although IIRC a few lines in a few places like Paris and Moscow do 85 secs.