You cannot directly change your allocated seat through the Trainline app or website once a booking has been confirmed. Trainline acts as a third-party retailer, and once the ticket is issued, the seat reservation is locked into the specific train operating company's (TOC) system. However, there are workarounds: for many UK rail providers (such as LNER, Avanti West Coast, or GWR), you can take your booking reference to the official website of the train company you are traveling with and use their "Manage My Booking" tool to view or amend your seat. If the train company's website doesn't allow a digital change, you can often go to a station ticket office and ask a staff member to check for available seats and issue a new reservation slip. Additionally, on many trains, you can simply move to any unreserved seat once you are on board, provided it doesn't have a physical or digital "Reserved" tag. In 2026, most "Advance" tickets come with a compulsory reservation, so if you are traveling in a group and want to sit together after booking separately, the most reliable method is to contact the TOC's customer service directly via their social media or live chat.