You can buy your ticket online before you travel on the GWR app
To buy your ticket in person, if you’re travelling from a station that has a ticket office or ticket machine, you must buy your ticket before you board the train.
If you’re getting on at a station that doesn’t have these facilities, buy your ticket on the train from the guard.
If you’re travelling from further afield, you should buy your ticket
in advance to get the best price. Tickets can be bought up to 12 weeks in advance. The earlier you book, the cheaper it can be. Buying a ticket for a long distance journey on the day will mean paying the walk up fare which can be very expensive.