Has any railway network managed to get less than 15 minute headways? Metros don't count, they're isolated and often enclosed.

Jup, quite common in the Netherlands. There are 10 minute trains from Utrecht to Amsterdam. And form Rotterdam and Den Haag to Schiphol. And from Utrecht to Den Bosch and Eindhoven.

Most of these are double decker trains and long platforms so they move a lot of people at once.

The highest frequency city pairs I can think of, at peak periods, looking at available tickets this week:

Shanghai Hongqiao to Hangzhou East is about 10 high speed trains per hour, all trains using the same line.

Tokyo to Shin-Osaka is also about 10 high speed trains per hour.

Taipei to Taichung is 8-9 trains per hour, high speed + conventional. Shanghai to Suzhou is similar.

Rome to Florence is 6-7 trains per hour.

Hong Kong West Kowloon to Shenzhen North is 6 high speed trains per hour.

Beijing South to Tianjin is 5-6 high speed trains per hour.

Most of Tokyo's mass transit network is absolutely neither isolated nor enclosed, and operates with vastly higher frequencies.

Here's is the timetable for a suburban station on a commuter lines: https://train-cloud.navitime.biz/en/odakyu/railroads/timetab...

On a weekday at peak hours, there are up to 20+ trains an hour, with commuter trains continuing directly into Metro systems, and directly onto different commuter lines on the other end.

At some point we had 10m intercity intervals between Rotterdam/utrecht and Utrecht/Amsterdam in NL.

Seems like it's 4 per hour on Rotterdam/Utrecht, seems similar to Geneva/Lausanne with 6 per hour.

In any case, I think commuters are fine with every 15 min, as long as there's enough seats. (for long distance like trains, my feeling is that frequency below 15min doesn't have a lot of impact, unlike shorter distance public transport like tram/bus/subway)

The Tokaido shinkansen has as low as 3 minute headways at peak times.