More than 400 rail passengers were refused travel for not wearing face coverings in a single week, latest figures show.
Transport for Wales (TFW) figures said 78% of customers are however complying with rules to wear masks on trains.
It has been law to wear coverings on all public transport in Wales since 27 July.
TFW said it refused travel to 426 people between 3 and 10 September for failing to comply.
Passengers must wear face coverings in all enclosed spaces, including in stations, on platforms and car parks.
TFW said it was working with British Transport Police to enforce the rules and providing extra advice to passengers.
Leyton Powell, safety director, said: "Statistics reveal that we have now positively managed behavioural change on our train network."