[dataengineer56]

This is UK law(?) or standard across all rail networks. It's automated too, you don't have to fill in a form. Immediately after arriving at your destination you get an email telling you that your train was delayed by enough to give you compensation, click a link and the money is in your bank account in a couple of days.

[lozenge]

Automatic Delay Repay depends on your train company and which company you used to buy the ticket.