Railway - tunnel or subway ?

Hi - thought I’d introduce myself. I guess I’m an ‘intermediate’ mapper of OSM. I was obsessed by it for a while last year, lost interest, and now I’m back for more.

My question is regarding which one out of [railway = rail] and [railway = subway] , is most appropriate for Australian inner city underground rail lines. I’m a train driver for CityRail in Sydney, so I have a particular interest in this and I have access to a lot of information about the rail system and I’ve also been recording GPX tracks across Sydney’s train lines - unfortunately this doesn’t work underground.

Tell me if I’m on the right track (no pun intended) - but from the research I’ve done, I get the impression that the ‘subway’ tag is more appropriate for commuter rail “metro” type services. I say this because it defaults to underground, and there’s an option to set a colour to define the line - and this probably suits systems like London and New York where different routes mostly run along their own unique tracks. I don’t think this makes a whole lot of sense in the Australian context given that Sydney and Melbourne’s inner city underground lines are shared by multiple routes, and you can’t set multiple colours to use the same track - so that whole idea falls into a heap.

Plus if one uses ‘subway’ as the tag, this line colour classification should extend out into the suburbs - in which case you’d need to make the entire length of the line as having the subway attribute, and I believe with the explicit [tunnel = no] attribute added to specify an overground subway.

Therefore I’m thinking the Sydney CBD rail lines need to be tagged as simply [railway = rail] and [tunnel = yes], rather than use the subway attribute I used when I dabbled with things a while ago?

And I’m guessing it’s okay to set [railway = subway_entrance] for nodes indicating street-level access?