On a train, the customers are in one place for significant amounts of time between stops. That place should not be where the guard is also located.
I don't follow this. If it's a long distance limited stop service e.g. Manchester to London then the guard would only have to emerge to do the doors around 4 times during a journey lasting over 2 hours, so even if someone is on the train for the entire journey they'll only be a few minutes of them being in close contact with the guard. On the other hand if the train is making lots of stops then it's more likely the same seat would be occupied by different passengers on the same journey, so it's a bit more like the supermarket example where the exposure will be with lots of different people for a short time.
If the guard didn't have a staff only cab to retreat to then it might be a different matter.
When I travelled from Manchester Piccadilly to Windermere I found there were crews changed 3 times en route, with one appearing to only do the Manchester Airport to Manchester Oxford Road portion of the journey. I don't know if that still happens but if it does it will certainly reduce the amount of contact time between the guard and any passengers making longer journeys.
Access still has to be given to the toilet on request unless the unit is coupled to another gangwayed unit. Passengers should not be allowed to queue for the toilet.
Last year Northern were saying they aren't making hand santiser available on stations or on trains but were encouraged passengers to:
a) Wash their hands more frequently using existing facilities available at stations and on trains.
b) Inform a member of staff if the toilet has no soap available.
I'm a bit confused how a passenger is supposed to do that if the guard is separated from the passengers and the toilet might be cordoned off.
The passenger human is choosing to take the risk by travelling on public transport and probably only will only be doing this once or twice during the day. The guard human is doing this for every stop and with shifts lasting hours.
So if a passenger has no car they are choosing to go to work but a guard is forced to work?
Another way of looking at it is crews often don't have to travel to work on trains with other people, so any potential exposure is only during their shifts, while the passengers on the trains might be exposed to others both on the train and while at work.