Some operators may not have the Delay Repay scheme, but details of their own compensation schemes should be outlined on their websites.
The train operator will usually need your journey details and proof of your train ticket to process your claim, and it will be checked against live train time information to calculate how much compensation you are owed.
Many train operators have online forms you can fill out to make a claim, but wait times for a response will vary depending on the operator.
You may also be able to write to the train company directly. Make sure to include your original train tickets, and take a photo or scan in case your letter is lost, when requesting compensation via post.
source: express.co.uk