I’m a new Mondo card holder up in the grim North and also work for one of the train operating companies as a driver.
Not sure if this problem will have cropped up before but most train companies onboard ticket machines were built in the early 2000’s and therefor have no ‘online’ facility at all. This means Mondo cards will decline if used on board. Some of the bigger operators can override by ringing the bank for an authorisation code but the local operators and indeed my train operating company can’t do this.
The work around is to ask the conductor to ‘Swipe’ the card which should work for transactions under £20 as it uses some old technology which I don’t understand and bypasses the need for chip and pin. However you will need to have signed the back of your card (you won’t believe how many people don’t).
The conductor may treat you with some suspicion however. This is a known fare dodging technique with some undesirables so your best bet if you board at an unmanned station is to use cash or dare I say a legacy debit/credit card.
From a manned station you should continue to buy before you board.
Hope this helps the Mondo team as it probably doesn’t happen in the south with your shiny trains and fully manned stations!!! Lol!