Travellers on the main rail service linking the Westcountry to London are set to experience major disruption in the New Year as part of the line closes for three weeks.
The main line between Exeter and Taunton will close between January 18 and February 9, First Great Western has announced.
Work includes track renewal, improvements to drainage and repair to the Whiteball tunnel.
Journey times will be increased by up to 75 minutes on some services with buses replacing trains between Taunton, Tiverton Parkway and Exeter St Davids.
Some services on the route will need to be diverted through Yeovil with train times altered.
A spokesman for the train operator said: "Network Rail will be carrying out essential works in Whiteball tunnel between Taunton and Exeter, they will also use the opportunity to carry out major track renewal and drainage works on the route. The line between Taunton and Exeter St Davids will be closed from Saturday, January 18, to close of service on Sunday, February 9. The works are needed to minimise the risk of delays and service disruption."
Des Hannon, Liberal Democrat Devon county councillor for Tiverton East, who uses the train to travel to Canary Wharf in London, said: "Quite a lot of people, the backbone of the economy locally, earn pretty big incomes in the City. "On the whole this area is exactly the same as the rest of the Westcountry, hardworking and low paid, but the people who earn more money in the City, spend some in London but the vast majority in the Westcountry.
"But you mustn't mess with that and you mess with it if people begin to think the service is unreliable.
"We need to understand why it's happening and we need to know it's not going to happen again – not for at least another five years."
Passengers travelling from Penzance to London Paddington will have to leave up to 65 minutes earlier to arrive in the capital by the same time.
The 5.05am service, will leave the West Cornwall station at 4am, during the closure, arriving in London at 10.02am.
But First Great Western said the diverted trains and re-timed morning service will only apply between Monday and Friday when the line is closed.