Airport trains

After reading the Wikipedia articles of the systems in Atlanta and Denver, I think that both can be considered subways (railway=subway).

The system in Atlanta is a normal subway using steel wheels on rails made of steel. The only distinction from other subways is that it runs only with in the airport.

The system in Denver can be tagged as railway=subway as well although it uses rubber wheels on a concrete track. There is no special tag in OSM for these "rubber trains" and even a couple of Metro lines in Paris use railway=subway although they are rubber trains only.

However, not all people movers at airports are subways. For example, Düsseldorf airport uses a monorail and railway=monorail would be the correct tag.

railway=light_rail would apply if the system crosses streets without bridges or tunnels. railway=tram would fit if the trains would run on the street and not have their own carriageway. However, the boundaries between subways, light rails and trams are not sharp. But that's a different topic …