Same in taiwan. I think they must have cellphone antennas underground

One of the deep level lines on the London Underground (Jubilee line) has 4G. It’s proved by a “leaky feeder” apparently.

I know subway in Warsaw does have them.

We're always going to have situations with intermittent network connections. It's best to design for that from the first. An offline-first experience is good even if you have a good network connection: lower latency (read from disk) and lower energy (the phone radio will be on for less time).

That comes at the cost of increased unnecessary bandwidth and battery usage though.

As a developer I'm much more interested in optimising for that than accommodating a minor improvement to the experience when experiencing connectivity issues.

The infrastructure of Asian capitals is the best in the world, that's exactly the point - 99% of the world experiences deteriorated network (see comments in this thread to that point). It's frustrating when software isn't built in ways that are resilient to that.

Most western European and all Australian cities I've traveled in had perfectly fine connectivity on underground trains.