Passengers will enjoy LED lighting, USB charging, a new live information system and a refurbished interior.
The Class 395 high-speed fleet travels at 140mph and connects London St Pancras International with Ashford, East Kent and Medway.
Unit 395012, which has a distinctive rainbow painted on the side, entered the train operator's Ashford train maintenance depot on 12 June to have interior refresh work carried out, and emerged from the workshops today. It will be followed by another 28 trains over the coming months.
The revamp will also include an upgraded CCTV system and energy meters designed to help Southeastern reduce its energy and environmental impact.
This upgrade is being carried out in partnership with Hitachi Rail and Eversholt Rail.
According to a survey from Transport Focus last year, Southeastern was rated roughly in the middle for passenger satisfaction compared with other operators. C2C had the highest satisfaction rate (95 per cent), Southeastern had 86 per cent satisfaction, while the worst performing was West Midlands Railway at 78 per cent.
Southeastern's engineering director, Mark Johnson, said: "Southeastern is committed to delivering better, more reliable and more sustainable services, supported by the fastest domestic passenger trains in the UK.
"With this refurbishment we're going to make the experience for our passengers even better. Carpets, seats and sides have all been stripped down, repainted or replaced, with a whole new look and feel, improving the accessibility of the train at the same time for all of our customers.
"This is just the beginning of the programme, as later in the year we're upgrading the passenger information systems on board all 174 carriages in the high-speed fleet, which will bring more benefits to our customers as we improve the way we give people the most up-to-date information about their journeys."
Jim Brewin, chief director at Hitachi Rail UK, said: "We are extremely proud to oversee the iconic Javelin fleet interior refurbishment and digital upgrade.
"Adopting Hitachi's infrastructure monitoring solution on Javelin trains will automate overhead line inspection to minimise disruption and reduce maintenance costs, while ensuring the fleet remains at the cutting-edge of UK train innovation."
In June, train operator Northern announced plans to equip as many as 40 of its trains with technology that will allow them to automatically track the state of the rail network.
