New disabled-accessible toilets will be installed on all 96 of the trains, along with improved seating arrangements for passengers with reduced mobility.
The changes are being made to make sure the trains, which were built in the early 1990s, comply with new regulations stipulating that trains must be accessible to disabled passengers by 2020.
Doors will also be overhauled and the trains repainted as part of the project.
While Chiltern’s trains are expected to remain in use between London Marylebone, Bicester and Banbury well into the 2020s, FGW’s trains are due to be displaced from services between London, Reading, Didcot, Oxford and the Cotswolds in 2016-17 by new electric trains.
Some are likely to operate on the new East-West rail link between Oxford, Bicester and Milton Keynes, from its opening in 2017 until 2019, when electric trains will take over, but most of the fleet is expected to be transferred to routes in the West Country.
Kevin Tribley, chief operating officer of Angel Trains which leases the Turbo trains to the two operators, said: “This contract demonstrates our commitment to the enhancement of our assets throughout their life.
“The vehicles will be the first of Angel Trains’ fleet to conform to the regulations on accessibility.”
Chiltern’s Clubman trains and FGW’s fleets of 125mph Class 180 Adelante trains and High Speed Trains already have disabled-access toilets.
The overhauls will begin in September this year and run until April 2018.