I offered to pay by card on the spot but the machine was not working.
Did the machine fail to process your card only or did it fail to process all cards? I am assuming that we are talking about the on train
machine carried by guards or other revenue staff?
Thinking about your journey did you have an opportunity to purchase a ticket prior to boarding your train
or was the station unstaffed with no operating ticket machine?
You will, no doubt, be aware of your responsibility to hold a valid ticket prior to boarding a train, where facilities exist (and are in working order).
With regard to your card - contact your bank to see if you have "off line availability" on your account (if you have the
contactless payment logo on your card it will probably be "off line"). If you don't, ask your bank to upgrade your account if possible - if not then make sure you have enough cash on your person if you want to purchase tickets on board a train as this will save you getting any further grief
like wot you have just experienced. Cards without off line authorisation are generally
not accepted on board trains.
N.B. If you have not paid the basic fare then matters can escalate even up to being summoned to attend court if this remains unpaid - you can nip this in the bud by paying the demand as and when it comes. You may have the right to appeal depending on the circumstances of your case.