Indeed.
3 minutes in the normal planned headway for classic intercity lines, TGV lines and Japanese Shinkansen.
90 seconds on metro lines with moving block, 2 minutes on standard metro lines.
90 seconds gives you about enough time to run a train in, get passengers to board, and have another one to run straight in behind it.
Parts of the London underground have traffic levels of ~ 40,000 people per hour. Moscow and Hong Kong metros have double that. You need very dense population to achieve the latter throughput and is not applicable to UK conditions.
HS2 is designed I think to run 18 trains per hour with 1100 on, so 19800 per hour. The Pendolino fast line service is ~ 5000 per hour, so nearly 4 times the traffic volume of the fast line.
http://www.hs2.org.uk/assets/x/77048