Swindon to Kemble railway work shuts line for three weeks
The railway line between Swindon and Kemble has closed for three weeks to allow engineers to upgrade the route to a two-track operation.
It is part of a £45m project ahead of the Great Western Main Line's electrification.
Signals and other equipment will be moved to make room for 12 miles (19.3km) of additional track.
While the work is under way a replacement bus service will operate between the two stations.
Railway operator First Great Western (FGW) said trains from Kemble to Gloucester and Cheltenham will run to an amended timetable.
Some peak services from London to Gloucester, Cheltenham Spa and Worcester Shrub Hill will be diverted via Bristol Parkway.
At other times customers will need to change at Swindon for the coach service to Kemble.
"Customers can expect journeys to take between 30-55 minutes longer than usual. FGW tickets will be permitted for use by other train companies," a FGW spokesman said.
The work, which involves some 600 railway staff, is due to be completed by 05:00 BST on 2 September.
The line between Swindon and the Gloucestershire village of Kemble was "singled" in the late 1960s as a cost-saving measure by British Rail, under agreement with the government.
Adding the second track is expected to allow up to four trains per hour to travel on the line in each direction.