The trains are leased to Great Western Railway, which will take over the operation of Heathrow Express from Heathrow Airport when the £11m refurbishment project is completed in December.

The upgraded trains will replace the fleet of CAF/Siemens class 332 EMUs, which have operated Heathrow Express services since the rail link opened in 1998.

The conversion of the 12 EMUs from suburban to airport configuration includes installing better-quality seats, power sockets, TV screens and Wi-Fi, and providing additional luggage space.

Porterbrook says this is the first existing British train fleet to be fitted with ETCS.