Yes. The trains you travel on must call there though. Of course if you change trauins there then that's the same.
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For example, let's say I am travelling London Euston to Birmingham New Street, with London MIdland as that's the cheapest. Let's say that if I buy a ticket from London to Northampton, and then Northampton to Birmingham, it's cheaper than buying one from Euston to Birmingham' (it isn't, but for the sake of this demonstartion, lets say it is) As long as the train I get on at Euston calls at Northampton - LM's Birminghams all do, then splitting the ticket there is fine.
Let's say that I decide to do the same journey, but split the ticket at Watford Junction. However the train I board at Euston is first stop Leighton Buzzard, and doesn't stop at Watford, I cannot split the ticket there as the train doesn't stop there.
That make more sense??