These Siemens built 4-car sets are used exclusively on the Paddington-Heathrow Airport 'Heathrow express' service and are the only electric trains to operate out of Paddington over the former Great Western main line. The route from Paddington was electrified expressly for the introduction of this service and it is planned to expand the airport connection via North London in the future.
Each of the Heathrow Express sets has part of one driving car set aside for the shipping of customs cleared luggage between Paddington and Heathrow, otherwise accommodation is provided for standard or first class travel. The sets are fitted with Spartan but comfortable interiors for the 15 minute journey between London and the Airport.
The sets are maintained and housed in the railway complex at Old Oak Common when not in use.
Advertising livery is applied to one driving car of each set under year long contract deals. The first company to use the trains to advertise their product was Vodaphone and from late 2001 the Royal bank of Scotland took over the advert contract.