Transport for Wales said more than 500 people were stopped from travelling on its trains because they were not wearing a face covering.
The figure covers the past week as enforcement was stepped up by staff and officers from British Transport Police.
Since July 27 it has been mandatory to wear a face covering on public transport in Wales.
Transport for Wales (TfW) said they have been encouraging customers, who are unable to explain why they are exempt from the requirements of the regulations, to wear a face covering for the protection of others.
Exceptions includes children under the age of 11, people who experience breathing difficulties, or those with a physical or mental illness, impairment or disability.
TfW is asking all customers to plan their journey in advance, maintain social distancing and not to travel if feeling unwell.
Leyton Powell, Transport for Wales Rail Services Safety and Assurance Director, said: “The safety of customers and colleagues is our top priority. From July 27 we have deployed the 3E approach of engage, explain and encourage as we recognised the benefit of wearing face coverings in reducing the spread of coronavirus. This helps us to keep everyone safe who uses our services as the lockdown measures are relaxed.
“We are not seeing the levels of compliance on our services in all areas so we have had to make the tough decision for the safety of our customers and colleagues to move to step up our enforcement activity. In partnership with the British Transport Police we will be refusing travel for people who are not respecting these rules. This could also result in some travellers being asked to leave the train at stations before their intended destination and even fined by British Transport Police.”
Signs and cordons will remain in place on Transport for Wales trains and stations until further notice.
BTP Wales Chief Inspector Jon Cooze said: “The requirement to wear an appropriate face covering on all public transport services is an additional safeguard to enhance the requirement to maintain two-metres social distancing where it is possible to do so.
“BTP officers are on patrol across the network and will continue to work with TfW staff to help ensure all customers have a safe and confident journey. We are nonetheless prepared and determined to use the penalties available in the new Coronavirus Regulations where there is non-compliance or blatant disregard for these safer travel rules.”
