The electrification of a number of routes in the south is running up to four years late, Network Rail has revealed.
The project, which was last year revealed to be going significantly over-budget, climbing from an original estimate of £874 million in 2013 to £2.8 billion now, is facing delays of varying lengths.
In Bristol Temple Meads, the arrival of electric trains has been pushed back until 2020 - some four years late.
Didcot, Newbury and Oxford will now be equipped with electric trains in 2017, 2018 and 2019 respectively, falling between two and three years late.
Speaking about the delays, Network Rail's Julian Burnell described the project as "very large and complicated", involving "a vast number of variables across a very large area of the country".
Network Rail's extensive engineering works went more closely to plan over the Christmas 2015 period, when a new approach to railway engineering not only reduced passenger delays, but saved hundreds of thousands of pounds.