Firstly, assume it's genuine.
"I don't know what I did wrong"
Secondly, you do need to try to work out why they sent it. That ought to be possible to do.
Only then can you decide how to respond, if at all.
Can you think of anything in your rail travel that might be unusual?
Think about how the following will look to them:
- your address
- tickets you purchased, stations from and to (e.g. buying tickets for only one stop or multiple tickets with gaps in between, where it looks like you made a longer journey)
- any ticket refunds that were made
- any delay repay claims that were made
- when those tickets were scanned, at barriers or on trains (e.g. unlikely or impossible situations - delay repay claims at different times from when the ticket got scanned)
- any railcard discounts you used with those tickets (e.g. job centre discount that's frequently fraudulent)
- whether those discounts were valid at the times the tickets were scanned (e.g. minimum fares, where you buy a ticket at a price only valid after 10am but use it before 10am)
- whether tickets were regularly purchased after boarding trains (e.g. the law normally requires you to buy before boarding)
- scanning tickets that are valid for a single journey in ways that look like multiple journeys were attempted
- buying a mixture of singles and returns for the same journey
Does that list give you any ideas?