I’ve got a season ticket from A Cardiff valleys station to Newport.
I’m going to Bristol today.
Can I just get a ticket from Newport to briston on the train or do I have to buy in advance from the ticket office in Cardiff or break my journey at Newport?
The only way you can be
sure you will not have to delay your journey or be in legal trouble is to buy a ticket before you board the train at Cardiff, e.g. at the ticket office/machines at Cardiff (or your Valley Lines origin station, if it has ticketing facilities), or using a mobile app.
If you are running late you can of course travel to Newport but before you continue onwards from there you must previously have obtained a ticket. It may be worth seeking out the guard onboard the train, as they will usually (though not always) sell you a ticket for a part of the journey you've not started yet. If the worst comes to the worst you'll simply have to get off at Newport and get the extension ticket there. Alternatively you could buy a ticket on a mobile app (not that I am usually in favour of this) between Cardiff and Newport,
and make sure to activate it before the scheduled departure at Newport!