The investment will see fully accessible toilets, additional wheelchair spaces, door modifications and toilet retention tanks fitted to the entire Class 158 and 150 fleet.
Once completed, the programme will make rail travel even more accessible for customers in Wales and England.
The Cambrian line, which is exclusively used by Class 158 units will become the first in Wales where no passenger service toilets flush onto the tracks, which will be a fantastic environmental benefit.
To make that possible, Welsh Government is committing nearly £200,000 towards new equipment for Machynlleth Traincare Depot to empty those train retention tanks.
Arriva Trains Wales Managing Director Tom Joyner said: "We are passionate about making rail travel accessible for everyone, no matter what their mobility needs.
"We have worked hard do deliver a comprehensive plan for these trains and customers can look forward to enjoying these benefits soon.
"This investment will go a long way to achieving those goals and shows a commitment to leave the rail industry in the best possible position at the end of the Arriva Trains Wales franchise."
The programme is being started by Arriva Trains Wales in partnership with Welsh Government and Transport for Wales and will continue into the new franchise.
Economy and Transport Secretary Ken Skates AM said:
“We are pleased to see the work starting on the improvement of the passenger facilities on these trains, ensuring all 120 carriages meet the accessibility requirements prior to 2020. Our investment into enhanced facilities at Machynlleth depot will ensure that the class 158 trains continue to be maintained locally to the route and passengers they predominantly serve.”
Arriva Trains Wales' fleet and train planning teams and partners in Welsh Government have developed a careful plan to minimise the impact on capacity whilst these vital improvements are made.
Stephen McGurk, Porterbrook’s Chief Commercial Officer said: “Porterbrook is investing over £10m in accessibility improvements to our fleet of 36 Class 150 trains operated by Arriva Trains Wales.
“We are particularly pleased to be able to involve the local supply chain in this work with the brand-new, fully-accessible, toilet modules being made by ‘Precision Custom Composites’ of Cwmbran.”
Malcolm Brown, CEO Angel Trains: “We are pleased to see work get underway on the modernisation of the Class 158 and 150 fleets. We are committed to working with the industry to make rail travel more accessible for all passengers and lessen its environmental impact.”