You can use either Apple Wallet etickets or PDF etickets.

When you get to the ticket barrier, you can scan the eticket from your phone, or you can print out the PDF eticket and scan the printout of it.
Etickets are available for offline use as soon as they are purchased through the app. But don’t forget to give your phone plenty of charge before travelling, as you need to be able to show your eticket at any point on your journey.