Electrification of the Great Western Main Line to Chippenham and the South Wales Main Line to Swansea was completed in 2020. The Department for Transport has postponed indefinitely the electrification of Bristol Temple Meads to Chippenham and Bristol Parkway.

Timeline
January 2020
Trains run under electric power for the first time between Cardiff and London. Network Rail Wales and Western managing director Mark Langman says he is
“delighted that services can now run on electricity between Cardiff and London, linking the two capital cities for the very first time via an electric railway.”
FoSBR wait to see whether deferral (rather than cancellation) of electrification means that wires will come to Bristol Temple Meads soon, either via Bath or via Filton Bank. The Network Rail website says:
Electrification in the area has been deferred until the next Control Period (CP6), which runs from 2019 to 2024
Late 2019
The electrification of the GWML via Chippenham to Thingley Junction nears completion.
June 2019
Network Rail admit that engineers have an “issue” electrifying the SWML Severn Tunnel towards Newport and Cardiff. The kit holding electric wires above the track in the four-mile tunnel is failing because of the salty conditions under the Severn Estuary. Specialists in electric rail are needed to solve the issue.
Network Rail’s chief operating officer Alison Thompson says she is “confident” the first electric train will run between Cardiff and London early in 2020 – after construction and testing takes place this autumn and over Christmas 2019 and New Year.
Hitachi IEP bi-mode trains travelling into and out of the tunnel will be powered by overhead wires but can switch to diesel to travel under the Severn Estuary if necessary. The bi-mode capability means also that the trains can switch to diesel where the wires end, ie, Thingley Junction or Bristol Parkway for Bath or Temple Meads bound trains.
January 2019
Trains ran under electric power for the first time all the way between Paddington and Bristol Parkway.
December 2018
Paddington to Swindon goes live.
January 2018
Trains run under electric wires from Paddington to Didcot as the electrification programme progresses west.
July 2017
Cardiff to Swansea electrification cancelled.
November 2016
DfT defers the electrification of the GWML from Bath Spa (Thingley Junction) to Bristol Temple Meads and also defers the connecting electrification from Bristol Parkway to Bristol Temple Meads station (Filton Bank).
November 2015
Department for Transport (DfT) defers the electrification of the SWML from Cardiff to Swansea.