And you're in very close proximity for a very long time. I don't know how HVAC works on an aircraft but I assume there's a large amount of recirculation.

I was on buses and trains this morning. They weren't nearly as crowded, the trips were a lot shorter, the air moved around at every stop, and like you said, they're all pretty local, so low risk of someone importing weird diseases. At least on the subways, you should still wear a mask if only because of the air quality. There's a lot of brake dust floating around.