The discussion was around 8-10 years ago, so I'm probably mixing it up with Seattle.
Scrolling around the UK with the transit layer active, I only see the complete metro/light rail/tram systems of London and Sheffield (incomplete/missing "heavy rail" for both). Newcastle has one metro line, but not the others. No other city's lines are shown even though the stations/stops are often mapped. (I checked Glasgow, Edinburgh, Manchester, Birmingham, Nottingham, Blackpool.)
In much of South London "normal" trains are the primary public transport. The stations are marked, the railways shown in light grey, and there are usually even timetables listed [1] but there's no "Transit" line shown. These lines and services aren't shown anywhere in the country.
[2] is a public transport view of OSM. The colours are poor, but the orange lines are mostly missing in Google.
[3] is easier to read but only a railway map, though over 90% of lines in Britain will have passenger service. I generally use this if I'm planning a flexible trip somewhere unfamiliar.
I see some long-distance lines are shown in Japan, though surely only a fraction of what exists. They also get a darker shade of grey, so there's been some local effort there. There are still missing local lines in Tokyo though. I know Copenhagen, and there's been a significant improvement recently! The city/suburban trains are now shown. The regional trains are still missing though, which includes the train from the airport to the city centre.
Native SE Londoner here. IMO Google is accurately reporting the transport in the area, it's not their fault that there is a shocking lack of it.
Lines like Victoria-Orpington and CharingX-Hayes are not the metro and probably shouldn't be highlighted in the transport layer in my opinion.
The solution you are looking for is TFL taking over the suburban southeastern lines. That's what happened in my adopted home of NE London and today they are orange lines on the map. We also got brand new trains and 4tph on Sundays!
Scrolling around the UK with the transit layer active, I only see the complete metro/light rail/tram systems of London and Sheffield (incomplete/missing "heavy rail" for both). Newcastle has one metro line, but not the others. No other city's lines are shown even though the stations/stops are often mapped. (I checked Glasgow, Edinburgh, Manchester, Birmingham, Nottingham, Blackpool.)
In much of South London "normal" trains are the primary public transport. The stations are marked, the railways shown in light grey, and there are usually even timetables listed [1] but there's no "Transit" line shown. These lines and services aren't shown anywhere in the country.
[2] is a public transport view of OSM. The colours are poor, but the orange lines are mostly missing in Google.
[3] is easier to read but only a railway map, though over 90% of lines in Britain will have passenger service. I generally use this if I'm planning a flexible trip somewhere unfamiliar.
I see some long-distance lines are shown in Japan, though surely only a fraction of what exists. They also get a darker shade of grey, so there's been some local effort there. There are still missing local lines in Tokyo though. I know Copenhagen, and there's been a significant improvement recently! The city/suburban trains are now shown. The regional trains are still missing though, which includes the train from the airport to the city centre.
[1] https://www.google.com/maps/place/Herne+Hill/@51.4521063,-0....
[2] https://www.openstreetmap.org/#map=10/51.4095/-0.1868&layers...
[3] https://www.openstreetmap.org/#map=10/51.4095/-0.1868&layers...