It's actually a policy applied by Merseyrail since the James St incident, but ironically it largely doesn't prevent issues because the vast majority of Merseyrail stations are not gated. Indeed I witnessed an incident almost identical to* James St at Christmas which was narrowly averted by the train stopping before the drunk nutter had chance to fall under it. The policy is controversial because typical poorly trained staff are applying it to people who have coordination issues due to a disability rather than those who have consumed excess (or any) alcohol.
Crikey, I got out of my car yesterday and pretty much fell onto it and stumbled away. I bet a Merseyrail gateline staff member would have had me arrested for drink driving. The reason was in fact that I'd just done a full day hillwalking and could barely stand up as a result!
* But slightly different from it; the drunk nutter only started leaning/banging on the train after it had been dispatched, so no member of crew had done anything wrong. He then fell up the footbridge (!) before wandering off.