Our place has stopped virtually all training, although they did at least manage to find a way for trainees who nearly had their hours in to get passed out.
My personal thoughts, relating to the situation generally:
Firstly, leaving it up to individuals to decide what they're happy with is, in my view, not ideal. Mentors don't want to let their trainees down. Trainees don't want to be seen to be being awkward. Many drivers just want to keep their head down and do as they're asked. There should be agreement at a high level, and clear guidance through both the union and the management.
Secondly, social distancing. It's not really about the personal safety of the two people in the cab - it's about limiting the spread of the virus for the greater good (e.g. if the mentor has it and passes it to the trainee, how many other people will the trainee then go on to infect?). We're then back to the need for clear guidance about what is and what isn't acceptable.
PPE? I've not yet felt the need to use any myself. It must be a clear barrier to communication though, whether that's between trainee and mentor or between driver and signalman (or other parties). Can anyone confirm or deny? If you're having to keep removing your mask so that you can be understood then replacing it afterwards (I've seen this happen), then you're probably defeating the object of wearing it!
That said, I recognise that we can't hide from this thing forever, and that the social distancing (etc.) measures are intended to control its spread rather than eliminate it altogether, so in turn it's about minimising contact rather than removing absolutely all risk of transmission. On that basis, you probably could make a case, generally, for training to be permitted, with exceptions for folk who do have a genuine concern. Again, one for agreement and clear guidance from the top of the tree?