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You can even get the PlusBus add-on ticket on the day, at the arrival station, providing it has a viable ticket office [I say "viable" since my experience of Brighton after its last revamping means that you need up to half an hour to do any face-to-face business there ... and that's after you've found where they've hidden the ticket office]. I've sometimes not known whether I'll want to use local buses somewhere until I see the weather when I arrive - so the flexibility of buying on arrival can be useful.