Bristol Temple Meads is closed for five days as they replace their 1970s signal box.
Network Rail says Easter is the best time for closures as there are less commuters, however the station will remain closed on Tuesday - a normal working day.
Bus replacement services will continue to run, with the closest station for London commuters being Bath Spa.
The Signal Panel Box is a system from the 1970s that controls the movements of all trains in the Bristol region.
It is operated by hand. The twist of a knob sends electric signals to the relay room, which then changes signal lights or alters rail tracks outside.
The upgrade will replace the control panel with a digital system housed at Didcot Parkway.
A third of the signal operators will lose their jobs, but Network Rail says the new, £130million upgrade will allow them to run more trains on the network at the same time, meaning more capacity for passengers.
- Find out more about the changes to the 1970s equipment below: