Trains cancelled over fake bridge collapse image

That leaves much more of a paper trail. People routinely are fined and jailed for pulling off such "pranks", partially because "fake threats"/"abuse of emergency response resources" are an exception to many freedom-of-speech laws.

A fake photo of a collapsed bridge however won't cross that criminal threshold.

If you create a fake photo/video with intent to cause disruption it absolutely crosses the threshold.

Intent is a valid legal concept. Certainly there's no way to try "swatting" without crossing that line of intent, but (for example) less-threatening prank phone calls can be in the grey area.

I presume there is established legal practice for handling these kinds of things, but for generative images the legal limits won't achieve wide awareness until some teenagers and assorted morons get hauled into court.

intent is very difficult to prove.

"I was just memeing, sir"

If you create one to prank your friend, and he ends up falling for it and sharing it in another group, and it gets to someone who alerts the authorities, without including the context of "this was sent to me by a guy who's a bit of a joker", and railway management's policy is to take all reports seriously rather than verifying their provenance...I find it hard to think anyone in that chain should really be held liable.

The person who alerts the authorities should be held liable - they had the option to verify before doing so, but chose not to.