The first completed Class 175 train rolled off the production line in Alstom's Technology Centre in Widnes, Cheshire this week with passengers already enjoying the benefits on the longer distance journeys.
The improvements have been described as a "significant investment", which will greatly improve customer experience.
As well as USB and plug sockets, customers will be able to travel on newly covered seats, with new carpet also being installed. Interior fittings including table covers will be replaced too, while the exterior will be rebranded in TfW colours.
All 27 trains within the Class 175 fleet will receive the investment, which will then be replicated on the other classes of train operated by Transport for Wales.
TfW Rail Services Customer Experience Director Colin Lea said: "It's fantastic to be able to deliver these improvements for customers, which they rightly expect to see on a modern railway network.
"We know being able to travel in comfort and charge devices on the go are incredibly important to our customers, whether they are travelling for 20 minutes or four hours, for business or pleasure.
"While we are investing more than £800 million in brand new trains, they take time to build and we want our customers to have a comfortable experience right now.
"So this significant investment is another major step to building a better railway for current and future generations."
The Class 175 investment amounts to £6.7 million and is currently scheduled to run throughout 2020. The rest of the £40 million to be spent across the rest of the TfW fleet in the coming months.
Piers Wood, Managing Director Rolling Stock and Services, Alstom UK said: “The Class 175s have been in successful service over the last twenty years, and thanks to our fantastic team at Widnes, will be back in service in top condition within twelve months. Widnes has enormous potential to grow and be a focal point for the rail industry in the North West and UK.”