I don't really agree with this, tbh. The law of negligence is as old as the hills, and it is usually a simple factual matter rather than anything more complex: would a "reasonable person" have done the same thing in the same situation.
If, as in the last case involving Merseyrail, the jury think that a reasonable person would have done a different thing in the same situation (e.g. not give two bells whilst a drunken girl was holding on to the train) then the jury will convict. Of course there's an element of hindsight in these matters, that's unavoidable, but the idea a jury or judge are not fit to make these decisions is wrong.
The reasonable person test should be an irrelevance, because we're talking about a guard who has been trained to operate in accordance with the operating procedures laid down by a designated body (MerseyRail, who hold a passenger train operating licence) and who is in turn authorised and regulated by the Government's appointed body, the Office for Rail and Road (Office for Rail Regulation, as was).
A reasonable person would, in all honestly, do as they've been trained and instructed to do by their employer, who holds a passenger train operating licence and who has been assessed as being competent to hold that licence by the regulator of the railways.
An untrained but reasonable person could easily do something that's highly dangerous or inappropriate for the railway, it wouldn't be unreasonable to do it especially as we're not requiring the reasonable person to be versed in operating instructions or trained in any way.
No, I'm sorry, but this needs to be taken out of the CPS hands entirely, and given over to ORR, and more appropriate tests carried out before prosecution is carried out.
As much as it pains me to say it, the guard currently is an irrelevance, I do not want CPS decisions on prosecution to dictate safety culture and policy on the railway, because they're not competent to decide on prosecution or the terms on which they'll prosecute. This should be left to the professionals, not amateur lawyer hour. I don't want workers being injured or killed, passengers being dragged under trains or significant damage to the railway itself occurring because our staff are dodging the CPS Perry Mason squad, and doing things in a way which is unsafe but keeps the CPS happy.