An entire fleet of trains has being taken out of service prompting the suspension of many routes after damage was found on wheels.
A chip was found in an Arriva Trains Wales’ train wheel on Wednesday, and further checks found “significant damage” to all 27 long-distance trains.
Services from South Wales to Manchester and North Wales are cancelled or operating a reduced service.
Travellers can use their tickets with other operators.
National Rail engineers are checking sections of track to see if the damage was caused by rail defects.
The line between Llandudno Junction and Blaenau Ffestiniog has been closed with a replacement bus service running between Llandudno Junction and Betws-y-Coed in Conwy county.
There will be further impact on capacity as trains are moved to different routes to keep as many services running as possible.
An Arriva Trains Wales spokesman said: “Customers should expect significant disruption, heavy delays and cancellations today to services on key long-distance routes in south and north Wales, and the Marches line.
“This is due to essential and urgent safety checks being carried out on certain trains within the ATW fleet and therefore all customers are strongly advised to check before they travel.”
Arriva also said that with heavy snow predicted for much of Wales, people should travel “only if absolutely necessary”.
Ben Davies, a spokesman from Arriva Trains Wales, added: “We’ll be trying to run the best possible service we can, and particularly to Cardiff-Manchester, north Wales-Manchester and the whole Wales route. There will be some bus substitutions, but of course the inclement weather coming in is not going to assist us.”
Ken Skates AM, Cabinet Secretary for Transport and Economy, expressed “deep concern” and has asked Arriva to provide replacement bus services for all affected routes.
“I will be raising this matter with the UK Government Department for Transport at a national level to understand how this situation has arisen, given responsibility for the rail tracks and points remains un-devolved,” he added.