In answer to the original question, all those many posts ago, and without paying too much heed to the inevitable 'post war' which has now broken out just for a change.... :roll:
A Guard accessing the driving cab whilst a train is moving, unless strictly for one of the reasons stated in the rule book - nearly all of which would be advised first to the the Signaller and to the TOC's Control - is risking his/her job. There are few Managers who wouldn't report it, and few TOCs who won't go through the procedures for it. And saying that you were just passing 'vital information' to the Driver, of that the Driver was a bit poorly and asked you to, etc etc, will hold no water, those reasons aren't valid and they won't buy it. It's one of the strictest issues now at most, if not all TOCs.
Whether that's right or wrong we can debate forever, but opening that door is a very, very foolish thing to do. It's perhaps more discreet on something like a Voyager or Pendo where, I presume, you are out of sight of both passenger and CCTV, but on a unit it's pretty blatant. Whilst some might argue that it poses no real safety issue, that the cab-to-cab is more of a distraction, or that shouting through the door for a request stop is better than using the official buzzer codes or whatever, the fact is that TOCs will hang you for it. There have been SPAD incidents in which unauthorised access to the driving cab has been sited as a contributary factor, and that is all the ammo that is needed to slap a pretty sturdy ban on it.
As for cab doors swinging open, it's not ideal and again a Comp Manager would certainly pick you up for it.
To the OP, it would if course be a tad unsporting to go around dropping people in it, but of course you didn't suggest you had thoughts on doing so, and from your suitably undetailed first post nobody will be tracing any guilty parties, so I fail to see why all the fuss about you posting, but hey ho!