Valley Lines operator Arriva Trains Wales is warning passengers to buy a ticket before getting on its train or risk a £1,000 fine.
The train company has launched a campaign urging passengers to buy a ticket where they can.
In a twist of unintended irony though, many of the posters have gone up at stations without a full-time ticket office or a ticket machine
The company has said it has recently served notice of a fine or prosecution on more than 500 rail users who have allegedly broken the law.
Offences included attempting to travel without first purchasing a ticket, claiming to have boarded the train at a station closer to their final destination than the station they actually used and adults travelling using a child ticket.
The company says fare dodgers are costing rail operators throughout the UK millions of pounds each year and that it is determined to ensure that paying passengers do not subsidise the cost of those who avoid paying for their tickets.
As part of the new campaign, posters will be placed at all stations and on trains throughout Wales and leaflets will be handed out to rail users which offer some top tips on how to purchase the correct ticket for their journey, saving them time, money and hassle.
Anyone caught without the correct, valid ticket can be prosecuted, face a penalty of up to £1000 or up to three months in prison.
Mike Bagshaw, commercial director for Arriva Trains Wales said: “The aim of this campaign is to help our customers to buy the best ticket for their journey and remind everyone about the implications of not doing so.
“Buying a ticket in advance of travel can save time and money. If a customer has not purchased a ticket in advance or there are no facilities to buy at the station where their journey started, only then should they buy a ticket on the train or from other rail retail staff.
“Most ATW customers buy the correct ticket prior to travel, but this still leaves a persistent few who do not and are therefore passing this cost on to all the other law abiding rail users. This is totally unacceptable and, as we know from our post bag every week, is not tolerated by our fare paying customers who expect to receive the best value for money from the tickets they purchase.
“It is the responsibility of the rail user to purchase the correct valid ticket for their journey and ATW is committed to providing the best possible value for money for all its customers by ensuring that those who don’t pay are caught, fined and potentially prosecuted.”
The Plaid Cymru politician highlighted the example of travelling home from Cardiff after a rugby international where no fare checks were made by train staff.
In the letter Mr Whittle wrote: “Whilst fare dodging is to be condemned, I’m sure you’d agreed that the company needs to ensure that it collects fares from passengers who are happy to pay but find they are not even approached by a member of the rail staff.”