Are you here to seek fares advise? There's a general introduction - a talk on bus and train fares I gave to the TransWilts Link meeting on 16th April 2016 - at
http://atrebatia.info/faretalk.pdf . Although it's targetted at Wiltshire, much of the advice is valid right across the Great Western network.
Have a read also of some of the threads on this board - they will give you lots of thoughts and may help answer many questions.
We have a number of forum members who know quite a bit about fares, what's available, and how they can be used within the rules. What I've learned from them forms the basis of that introduction, and if you post questions here they'll be happy to help you too - if I know or can reseerch the answer easily.
If posting a question, please provide as much background as you can. No need to answer everything but the more we know of these the better:
Where you're starting and where you want to end up
Dates of travel
Times of day and how flexible you are with those
Whether your plans may change
If and when you're coming back
The size of your party and any railcards held
Please bear in mind that advise is given to the best of my / our ability, but we're not lawyers nor trained on ticketing, so you need to double-check if in doubt, and we can't be held responsible if we get it wrong. Mind you, if we do get in wrong in public, another member will usually post to tell us - making the end result pretty darned accurate!
Penalty fares were recently extended to cover the whole Great Western network, and if you have an opportunity to buy a ticket before you join the train, you must do so or you may be asked to pay a penalty fare or be prosecuted. There aren't ticket machines at some of the smaller stations, and in that case it's OK to join the train without a ticket and pay the conductor. The same applies where the only means of payment at a station is credit or debit card, but you want to pay in cash.