I've been on the train a couple of times and not worn a mask. The only one not wearing one though unfortunately.
The conductor never even mentioned it to me, but if he'd have asked I'd have simply said that I have a medical condition. I think most of these conductors would feel awkward asking and would just assume you have a hidden disability or medical condition.
Not that it's any of their business if they do ask (not obligated to disclose my medical history), but my medical conditions caused by mask wearing are hypercapnia and anxiety. Both technically true ?