Rail passengers travelling without a ticket will be fined from Monday on lines where they could previously buy tickets on board.
As part of Transport for Wales' (TfW) penalty fare zone expansion, travellers on South Wales lines and the Welsh Marches line could be fined either £20 or double their ticket fare, whichever is greater, if found to be dodging their fare.
TfW said the expanded zone was to tackle fare evasion which costs it "millions each year" in revenue.
Alexia Course, chief commercial officer at TfW said the process was not to unfairly penalise people but to "identify deliberate acts of fraud".