1 July 2016 Last updated at 08:02 BST
The Hitachi Super Express was due to start testing a year ago, but despite being delivered on time, was delayed as the electrification programme to power it is more than three years behind schedule.
Fifty seven trains are now being fitted with diesel engines meaning they can go into service next year on the Paddington line.
The train, built in Japan, made its first run from Reading to London on Thursday.
A new high speed train for the Great Western Railway's services through the Thames Valley has taken to the tracks.