Muricu91
You mean all the security camera footage in the trains and stations, showing you with a timestamp wehen boarding and leaving the trains? Dream on, we’re not in China…
Your company calendar entries and maybe some additional mails, meeting notices, work protocols, etc. should be sufficient as an evidence/proof that you were actually travelling there and didn’t take some private vacation instead!
And assuming the GA is your personal one (paid by yourself) and you are entitled to a refund of the regular ticket costs: Simply get the ticket price for the same itinerary (same weekday) on the SBB online timetable for determining those ticket costs, and take a screenshot of that page as additional evidence to the artifacts I’ve mentioned above. Also check first whether you’re entitled to a full fare or only a half fare refund.