Network Rail has announced extensive engineering works over the Christmas and New Year period.
These works will affect services across the Wessex route, which includes parts of Surrey, Berkshire, Hampshire, Dorset, Devon, Somerset and Wiltshire.
The upgrades are intended to improve the railway and make it more reliable.
The works have been scheduled during the festive season when fewer people use trains, to minimise disruption.
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From Sunday, December 15, until Sunday, January 5, engineers will be upgrading the railway between Basingstoke and Woking.
This will result in no direct trains between Exeter/Salisbury and London Waterloo.
Services between Waterloo and Southampton/Weymouth will be diverted, adding 45 minutes to journey times.
There will be no trains running between Guildford and Farnham.
Network Rail will be renewing the switches and crossings at Brookwood Junction, where the line to Ash Vale and Aldershot leaves the South West Main Line towards Basingstoke.
Other works include strengthening embankments at Ashmead and Hook, reducing the risk of landslips, and improving drainage between Brookwood and Farnborough.
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The Sherrington Viaduct between Salisbury and Trowbridge will also be upgraded.
This will result in buses replacing trains between Trowbridge and Salisbury from December 24 to January 24.
Mark Goodall, Network Rail’s Wessex route director, said: "Passengers are advised to plan ahead this Christmas as we carry out these essential improvement works.
"Our teams will be working round the clock over the festive period to upgrade the railway for our passengers so they can benefit from smoother and more reliable journeys.
"We do a significant amount of works over the Christmas period as we can keep disruption to a minimum, but we are sorry to passengers who are affected by these works and thank them for their patience."
Peter Williams, customer and commercial director for South Western Railway, said: "Our colleagues at Network Rail are carrying out a number of improvements on our network over the Christmas period, so it’s vital that customers plan their journeys in advance.
"With some services on our network amended, diverted and replaced by buses, we recommend checking journey planners now.
"We’re sorry for the disruption this will cause and are grateful to our customers for their patience."
Jordan Welsby, GWR station manager for Westbury, Wiltshire Dorset, added: "It’s important that customers are aware these alternative travel arrangements will make journey times much longer, so check before you travel."
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