The Severn Tunnel is due to close for nearly six weeks from September, causing disruption for rail users.
Work to prepare for the £2.8bn electrification project, to speed up journeys between London and south Wales, will cause the longest closure of the tunnel for at least 50 years.
Rail users will be diverted via Gloucester or use replacement buses.
Network Rail told BBC Wales the work had to be done in one block and guaranteed it would not overrun.
Work has already been carried out to prepare the 130-year-old tunnel for closure with four tonnes of soot removed from the tunnel walls and 7,000 holes drilled into the roof. A conductor rail will be installed to provide power.
Network Rail said there would be fewer services while the tunnel is closed and added that the work would take four years if only done at weekends.
David Sidebottom, from the group Passenger Director for Transport Focus which represents rail users, said: "The important thing is actually hearing from passengers, and this is the beauty of social media these days.
"We'll be gathering that up and making sure Network Rail and train companies are listening to the experience on the day because there will be times when they don't get things right and it's important they learn from that and put it right for the following day."
Dan Tipper from Network Rail said: "We have a significant volume of work going on across the western and Wales routes, and we have to avoid disrupting passengers twice in one journey, so we have to pick a time when we avoid other works."
Electrification is expected to cut journey times between Swansea and London by 20 minutes.
The tunnel will shut on 12 September and reopen on 21 October.