Maestro cards are no longer accepted for on-board purchases, they can only be used at booking offices / TVM's that are not a remote / mobile terminals.
Cheques are not accepted on board, or at ticket offices unless by prior arrangement, I believe.
"Electronic use only" cards are not accepted on-board.
Pre-pay credit cards are not accepted on-board.
Solo or Electron cards are not accepted on-board.
Depending on the situation with a declined card, the location, the coverage, how busy the train is, and where the passenger boarded, most times now I will ring for authorisation, as it is as quick as ringing to confirm an address for a UPFN. If I am more than usually suspicious (to quote Judge Judy, "If something doesn't make sense, it's usually not true"), the bank declines the card, or they claim to have boarded from somewhere where they had an opportunity to buy a ticket, then they leave at the next stop to sort it out. Platform staff at the next stop will be called over to "advise" them where to go. If the bank declined their card, and I have time, I may issue a UPFN from their starting point to the stop they are alighting at anyway.