Having lived in Cornwall in the past I can tell you that, that train will be a nightmare! Lot's of suitcases if thats what you mean

Although, if you really wanted to guarrentee a seat. You could book a First Class Advance fare return for ^105.00 Which sounds expensive, but considering a walkup ticket is ^81.00 return and the journey is 5 hours... I think you may find that most to your liking

If you have a railcard, it will be cheaper

However, you could, if you prefer, take advantage of the Weekend First. But that is subject to availability and could work out more expensive than a native
FC▸ advance ticket.
I'm not sure on the policy of upgrading Standard Class Advance tickets on Weekend First... (Devon_Metro will probably know

) but you can book those for ^55.00 Return Standard Class

And to extend Chris' Post. Welcome to the forum
