Having worked in schools, just a few thoughts if I may. Bear in mind that safeguarding is a real "biggie" in a school and Ofsted will (and do) downgrade the school on just that aspect, even if everything else is considered excellent.
The "phone the school" thing is thus a bit of a red herring. Office staff wouldn't divulge info about individual children to a third party. They've probably gone home after 4pm anyway, and teachers wouldn't necessarily be able (or inclined) to answer (or hear) the 'phone ringing. There would be procedures for parents/carers to contact a school, but usually, it was us ringing them if something was amiss.
We used to get a visit from the railway once a year about trespass dangers, playing "chicken", etc. This seems to have fallen a bit by the wayside with all the separate TOCs around nowadays, rather than one central body. We also used to take one year group to a place where they taught the children about dangers on the road and railway - I remember they had a Pendolino cab there. But funding (or the lack of it) reduced all that. Maybe GBR will be more proactive.
Is safeguarding a part of training for guards/RPIs I wonder? Does part of the course include how to interact with children? Doesn't sound like it's sufficient or effective if it even does take place.
As many have mentioned, the response from SouthEastern was dire, let alone the attitude and behaviour of the RPIs. Something has to change.