I don't think there's much that I can add to Michael769's post from yesterday.
Only the Railway Company's Prosecutor and your husband's fading recollection know what is written in the Statement taken on the day of travel, and any advice by way of a response he can make at this stage, without sight of that Evidence, is unsound.
However, at some point, a Summons will be sent to him, and it will include not only the date and place of the Hearing, but should also provide an opportunity to Plea by post and, crucially, a copy of the Prosecution's Evidence. That will be the moment to make an informed decision.
Despite the understandable anxiety and stress that this is causing, I'll also repeat Michael769's assurance that a Byelaw Offence is not a Criminal matter, and is similar to, for example, minor motoring Offences. You pay the Fine and move on; no lasting record.
Its unlikely that there is going to be any grounds on which he can challege the simple fact that the ticket wasn't valid and that he had admitted enough to clinch the accusation - it might just be easier to Plea Guilty by post and accept the Fine (reduced for early admission). But every case can be different, and much depends on circumstances - a local Criminal Solicitor (without specialist Railway knowledge - thy charge hundreds, or more!) might just find the opportunity to negotiate a compromise settlement, perhaps by phone, perhaps on the day of the Hearing; such a settlement might just allow the Company to walk away slightly better off than if they allowed the Court to impose a Fine and Costs, and avoid the Prosecution.
I only write this in repetition of Michael769's points as he did suggest others might be able to elaborate further. At this point, sadly not. But when you receive a copy of the Prosecution's Evidence, then there might be more to be said.
Don't worry, do remember that it isn't a Criminal matter.