It is used on tram tracks (railway=tram) and also on light rail tracks (railway=light_rail) where the halt resembles more a tram stop than a railway halt (railway=halt).
but is such underground station really more like tram stop than subway/rail?
My personal opinion in this case is that station=subway would be completely wrong. This is no subway, this is a light rail. There are several tunnel passages but this is no real subway.
Quoting from
This is not true for this line. It’s using regular railway for the connection part between Cologne and Bonn, see Stadtbahn Köln – Wikipedia
This thread is confusing. At first, you (@Mateusz_Konieczny) criticised the use of railway=tram here, which is a reasonable thing to criticise. But then you suddenly suggested to change the value of station without stating any reason. subway does not indicate the location of a rail line or stop, it’s just the OSM term for a metro, which this isn’t.
well, if that is not subway then my guess was wrong
though that is why I created thread asking more local people to take a look rather than making edit myself - because while I can see that it is quite likely wrong, edit based on remote guessing would be unlikely to be helpful
would railway=halt would be more appropriate here, treating this as a light rail line, and this stop not being a typical tram stop?