Great Western Railway (GWR) has unveiled the first of a fleet of new trains on the 175th anniversary of the opening of the line between Bristol and London.
The Hitachi-built trains will boost capacity and lower journey times on routes between the capital, south-west England and South Wales.
Passengers will benefit from free Wi-Fi, LCD seat reservations and increased space in overhead luggage racks.
The trains will be introduced into service from next summer on the Great Western Main Line, and from 2018 on services between London and Devon and Cornwall.
Transport Secretary Patrick McLoughlin, who travelled on the new Class 800 Intercity Express Train from Reading to London on Thursday, said: "Britain's railways have a glorious past and even brighter future thanks to the record amounts we are investing to modernise the network, deliver better journeys for passengers and drive economic growth.
"The unveiling of Great Western Railway's first state-of-the-art Intercity Express Programme train offers a glimpse of the benefits passengers in the South West and Wales will enjoy from 2017.
"These include more seats, greater comfort, better reliability and faster, more frequent services."
GWR's managing director Mark Hopwood said: "On this special anniversary we are looking not just back at our rich heritage but forward at a transformational investment programme.
"The new trains will result in more frequent and faster journeys and an increase in the number of seats.
"These, and a range of further passenger benefits, will enable communities and businesses across the Great Western network to prosper."