Which obviously should take precedence over someone's personal device, I'd consider this in the definition of affecting productivity, but this kind of thing just isn't often part of the remit of most operational railway staff. I'd fairly confidently say that the majority of railway operational staff across the board do not have very many work emails to personally respond to on a daily basis, and reading briefing documents like a traffic circular or relevant training material isn't likely to take up all that much time at all. If you work in an office where your remit and training expands beyond a specific single task, or you're in a roving managerial position (which do exist) in which you genuinely can find things to be getting on with, even if it's essentially just networking, then that's great and it should obviously be prioritised over anything else (and in my experience, almost always is).
But the reality is, there genuinely are positions across the railway and wider transport industries, as well as many other industries, that require someone to be in one exact location for operational, safety or legal reasons but don't necessarily have productive work-related tasks to be undertaken throughout every minute of the working day. Providing they're still 100% able to jump straight back in and do their job when required, then being on their phone is essentially no different to just sitting there doing nothing and staring into space, or re-reading this fortnight's traffic circular for the 40th time. Whether that should be the case or whether it's down to inefficiencies of staff allocation or convoluted government regulations is another thing, but let's be real, that's unlikely to change and is a systematic issue rather than being the fault of the general operational staff member.