Transport for Wales has taken over the Wales and Borders franchise.
The new operator replaces Arriva Trains Wales who have run the service for the last 15 years.
Transport for Wales have promised a "transformation" of the service by 2025. They say that by 2023, 95% of passengers' journeys will take place on new, higher capacity trains.
On the first day in charge of the franchise, the new operators have begun deep cleaning of stations around the country. A new ticket website has also been launched.
After fifteen years in charge of the service, Arriva Trains Wales say they are "proud" of the service delivered on the franchise.
But some customers celebrated the end of the company's time in charge, a celebration was held at Cardiff Central Station the night before the new company took charge.
The transport watchdog, Transport Focus, has welcomed the takeover.
Some services have been affected today on the rail network as Storm Callum continues to cause disruption.