Long story short, I discovered the subway tag and several aspects of it bug me.

For background, I'm from a country with newer and lower-density cities than most other places. I've previously lived in my country's third-most populous city, and at some point have visited most of its train system, which is almost entirely above ground with the exception of a handful of tunnels and approximately two and a half stations.
But if I were to post an accurate depiction of a commuter train from that city in its natural environment, it would fall into the current visible standards of the subway tag.

I'd like to argue that subway as a tag is too subjective; we have train, inside_train (which most applicable subway posts were/are missing), and public_transportation (also missing from a lot, and needs to have public_transport aliased in)
There are as many posts showing city skyline views as there are showing tunnel wall(?) views. Many posts show so little scene information they might not even be on a train.
There are also predictably a handful of sandwich restaurant posts I've yet to clean up into subway_(restaurant).

I can see it being a useful tag for some aspects, potentially with a disambiguation tag.
subway_tunnel could be used for subway train tunnels and/or underground pedestrian walkways
subway_entrance could be used for the occasional depicted street-level subway entrance
While subway_train would not technically apply to all trains depicted within a tunnel, I'd be comfortable with a TWYS-based association assuming that a metro train in a tunnel is subway.

I've created ticket_gate for this post to retain searchability. Potential discussion of whether this needs to be merged with turnstile, which currently holds two posts. I've also assumed public_transportation is a reasonable association in this case.

I think that's all I've got to say for now. Anyone else?