My son has a ticket from Leuchars station to London. He will be in Edinburgh the day before his train leaves and is thinking of spending the night there. I was wondering if he could use the same ticket and grab the same train (on the same day) instead in Edinburgh that he was supposed to take from Leuchars (yes, the train does make a stop in Edinburgh). If not, he could easily go back to St. Andrews the day before and take the train from there.

It depends what sort of ticket he has. If it is an advance ticket, you are not supposed to break the journey: the ticket is only valid for the exact journey printed on the ticket. If it is an "anytime" or off-peak ticket, then you are quite entitled to catch a train from Leuchars to Edinburgh, then another train from there to London.
Whichever sort of ticket he has, if the seat is reserved from Leuchars to London and the seat is not claimed, then any other passenger is entitled to sit in that seat; if he joins that train at Edinburgh, the seat may therefore be occupied by someone else.

GeoffH: As I read the OP (and I may be totally wrong) He isn't thinking of breaking the journey in Edinburgh. He has a Leuchars > London ticket and only wants to board the train at its Edinburgh stop (having traveled to Edinburgh earlier)

Do I have that right?

If so, Shouldn't he be able to board in Edinburgh? It doesn't work like flights where you lose the whole booking if you miss the first leg.

There are plenty of reports of people being surcharged for getting off a train at an earlier stop, and I imagine boarding at a later stop may present the same problem.

British train tickets, especially cheap ones, are usually for specific journey between specific points. Leuchars may offer special discounts, or tickets with special conditions, and you should be wary of breaking those rules.

There are always fully flexible tickets available, but at a higher price. If you buy a cheap ticket, there will be restrictions.

Thank you for all your responses. I was able to access my son's account (he has no internet access right now).

Geoffhamer - His ticket is an advance one which does not allow for any changes.

janisj - Yes, you are correct but apparently hopping on in Edinburgh is not allowed since his journey is supposed to start in Leuchars. It doesn't seem to make sense, but rules are rules, I guess.

chartley - I've heard of those incidents, too, where they got charged a hefty surcharge.

He's changing plans now so that he'll go to Edinburgh a day earlier and stick to his original travel plans.

Thanks again.

In theory he should get a new ticket as officially he'd have broken his terms of contract.

However as there isn't any master computer system checking where people boarded or gates at Edinburgh stopping people getting on trains he should be able to use his ticket - unless a jobsworth "revenue protection officer" decides to follw the letter of the law.

alanRow - makes sense and he's telling me the same thing. Right now, he wants to stay longer in Scotland! He can change his tickets for an administrative fee. Whatever he wants; it's his money.

I do a general question. Is it better to buy an advanced ticket online from Leuchars to Edinburgh or just walk up and buy a ticket that day? I checked trainonline.com and they have a 1 pound fee and a 3.50 pound credit card fee. Ouch for a 7.40 pound ticket (with a railcard)! Not sure, though, what the walk-up ticket price would be.