The first of the Class 170 trains which have been cascaded from Greater Anglia, have already arrived at the Transport for Wales' Canton depot.
The three-car train has 186 seats and is set to be joined by a further seven three-car-units and four two-car units that each provide seating for 110 passengers.
The Class 170 Greater Anglia Turbostar 's are being displaced by brand new Stadler Class 755's that are running on the Norwich to Cambridge line.
The 170's will run on the Cheltenham to Maesteg line from December.
Transport for Wales is also considering using the 170's on its Ebbw Vale and Heart of Wales lines.
Rolling stock director Sara Holland said: “The Class 170s provide a more modern and reliable service with air conditioning and power sockets.
"These are comfortable trains that will significantly improve capacity and provide a high standard of on-board customer experience.
“With the support of our industry partners, our fleet colleagues are working incredibly hard to introduce new rolling stock, refurbish existing trains and carry out much needed modification works.
"This is an £800 million programme that will transform the experience for our customers and colleagues alike; the average age of the fleet will drop from 25 years to seven years by 2024 and 95% of journeys will be made on a new fleet of 148 trains by 2023.”