It's the exciting part of 5g everyone talks about despite its somewhat narrow application. I guess busy subway stations are a good fit as they have large numbers of people in a tiny area. And in larger stations you can deploy multiple cells with minimal overlap to split the load.
Even with regular 5G (sub 6 ghz) you'd take advantage of improvements over LTE like massive MIMO and more precise beamforming. All leading to more people using a network at the same time. Also anecdotally I've found that at music festivals, when cellular data doesn't work, texting or calling usually works fine (At least on AT&T)
