I was under the impression that "This train is delayed/terminating short/canceled due to a police incident" was standard practice already. If not, it really should be.
It makes sure people aren't blaming the railway (i.e. abusing staff for delays outside their control)
It is sufficiently "coded" that most will likely guess what it is, but is sufficiently vague that they can also imagine it's nothing quite so tragic but which adequately accounts for the disruption.
It doesn't invite queries on Twitter, and anyone who persists can be given the stock (and I would have thought entirely true) reply, that the company cannot give out details of ongoing police incidents. You cannot stop speculation, but it gives people little to speculate about.
It helps avoid giving a location, since how would that help anyone better understand the delay? You give out only what is needed to understand the alternative options, which on the raiway will typically be far removed from the actual location. E.g., all services past Station X are temporarily suspended due to a police incident.
Avoiding "emergency services" avoids needlessly having passengers distressed as they contend with the news, hopefully in blissful or at least conscious ignorance of why their journey has been disrupted. And it's not a lie, police would always attend an incident of this nature.
It obviously gives leeway for staff to be more explicit if a passenger really makes a fuss, without inviting anyone to really question what is going on. Just as you wouldn’t really question an actual police officer manning a tape where the cause of the closure is out of your view. If it is stopping your journey, you simply ask what are the alternative options, rather than asking why the tape is there.
It even helps parents. Children young enough to be seriously distressed by any remotely truthful explanation, and yet old enough to tell when an adult is keeping them in the dark (guaranteed to invite further queries), wouldn't bat an eyelid at a more benign reason why the police might be holding up the railway.