@Loui0se
If you have checked with Scotrail there was a train breakdown ( look up the Scotrail site for news of disruption) and if this is what has happened this evening, then....
I would refund them 50% of whatever your cancellation policy is for this one night only with, no more, further cancelled nights, as a goodwill gesture.
I would Phone the air bnb number in London and explain this is what you want to do,after their departure at the end of their second night stay.
Id let air bnb communicate directly with guest.
If they cancel both nights, then I would implement your cancellation policy in full unless more train cancellations occur due to Scotrail cancelling.
Its really up to you, whether you think they should have travel insurance, it’s nasty weather etc and decide accordingly.
Tonight the weather girl, Heather, is saying it’s “ blawn a Hooly on the west coast”....
the dour embarrassed soor puss brigade always hovering somewhere waiting to be offended by more...Och aye the Noo.....”Soz” but it’s even finding its way into, BBC Scotland.......Jings, Crivens and help ma Boab 😉