I speak as a (very) small employer, but the thing I don't understand about all of this is that if an employee is a good conscientious worker why would an employer want to get rid of them ?
On the other hand if the employee is not so good or conscientious then why should the employer be expected to keep employing them ?
Hard to comment on this specific case as details are scarce.
But in a more general sense, if the working relationship between an employee and an employer breaks down, that can be the end of it, and it's also a perfectly legal reason to dismiss an employee. Note though that there are 'automatically unfair' reasons for dismissal, which includes taking action or proposing to take action over health and safety issues.
What you would expect is for an employee to sit down with an employee and work out how to remedy the working relationship. Whether that happened here or not, I'm not sure, but I'd expect so as I doubt any train operating company (open access or franchise) would make rash decisions about dismissals without at least trying to fix an issue.
I suppose the question really is - did the employee and the employer end up in a deadlock over this safety issue? If the employee is adamant it's a critical issue to address, and possibly is even refusing to undertake some duties because of it, or causing significant disruption because of it; but the employer feels it's not an issue which needs addressing or not in the way the employee expects; and neither side will budge, they may have determined that the position is untenable.
I am assuming that if the union is backing the worker on this, that they believe the safety concern being raised is reasonably important, or that the employer hasn't taken all the right steps in trying to come to an agreement over it.