It's been touched on upthread but there are a number of wider industry issues with this announcement and with eight days and eight hours left to prepare for it it's no doubt going to prove somewhat challenging.
The rail unions have all come out as saying this requirement is a positive; their enthusiasm is not matched by some (many?) of the staff on the ground, who will, rightly or wrongly, quite simply refuse to work if they're forced to wear unsuitable kit. Do you blame them when their managers are buying their own gear because they have no faith in the ones the employer is providing? The obvious impacts of this would be resourcing of traincrew, but also it's worth considering train maintenance teams, station staff, management and support staff behind the scenes need to be in place to ensure a service can run. It isn't just a case of a driver jumps in a train and there you go, you're running a railway.
It's been said that the rule will be written in to the National Conditions of Travel, but this doesn't make it a legal requirement and in effect means the police will be able to wash their hands of dealing with it. TOC staff have generally (I don't know if there are any exceptions) been told they are not there to enforce the coronavirus control measures. So who will enforce the requirement to wear masks? If it isn't enforced, then I give it approximately three days before people realise everyone else isn't wearing one and aren't being pulled up on it so they'll stop.
Those, like me, who have underlying health conditions which make use of masks particularly challenging are currently in very much a 'limbo' as we don't know what the requirements are going to be. It's been said there will be an exemption, how will this work and what if by the time it is announced there will be little time to prepare? Clearly if a there's a depot of conductors who all suffer from respiratory issues are told with a couple of days notice they will need to obtain a doctor's note to not wear a mask, they are not going to be able to come in and work trains until they have been able to see their doctor to obtain one.
There's also no requirement for these masks to meet any form of standard, or if they do, fit the wearer correctly or be worn correctly. A properly fitting mask which is suitable for the given task will help protect against transmission. A here's one I made earlier mask made out of an unsuitable material and which isn't properly fitting achieves very little other than an illusion of 'look, I'm helping!'