I'm not disagreeing with FareCop or DaveNewcastle, and greatly value their contributions, but I do think that:
1) Each case would turn enormously on its own facts, for example, if the passenger knows there will likely be no further opportunity to pay after the intermediate station, then that would tend to support the view that that passenger is intending "to pay only if challenged"- .i.e. to evade their fare.
2) In general, having regard to the NRCoT, to the fact that the facts in Corbyn can be distinguished in so many ways from these situations, and to the relevant burden of proof, it would seem unlikely that failure to purchase at an intermediate station, where there was neither any prior opportunity nor any reasonable opportunity to pay at this intermediate station without delaying their journey, constitutes sufficient evidence alone to prove beyond reasonable doubt that the passenger intends to evade their fare.
(In principle, such a passenger is no more guilty of fare evasion than a weekend traveller who boards with a valid standard class ticket, sits in first class as permitted by the published T & Cs of the upgrade, and waits (and intends) to be upgraded at the first opportunity. If they are not upgraded on board, then that would seem to be inadvertent avoidance rather than deliberate evasion. Here, in the OP's scenario, again I would say that the evidence, and the wording of the NRCoT, tends towards the supposition that the OP, in boarding his/ her onward train, is fully intending to pay and has never deviated for a moment from that intention, but is merely acting reasonably so as not to unnecessarily delay their onward journey (which, as an aside, might potentially trigger Delay Repay).
3) However, when the fact of non-purchase at the intermediate station is used together with other evidence, having regard to all the circumstances, including the defendant's words and actions, it may be that the magistrates are satisfied beyond reasonable doubt that the offence is made out.