Trivia. The timetable, or Working Table, for all London Underground train movements is here https://tfl.gov.uk/corporate/publications-and-reports/workin...

Crazy that the time between trains is measured in seconds

Even with a strict timetable you need extra spacing between trains and potentially slowdowns to allow passing to be done properly. These systems on busy lines tend to run trains as close to each other as possible without causing safety issues so there's no buffer room. And if something goes wrong for any reason the whole system starts getting cascading delays.

> The tendency to not keep to a strict timetable and the close-running of metro systems makes it hard to slot express trains into gaps between local trains properly.

To be fair, "close-running" is not a word that tends to be associated with the District line ... "signal failure" is, however ! ;-)

The central section (including the Circle Line) has a service interval of 2-2½ minutes, depending on the time of day.

The rest has roughly 2-10 minute intervals.