A derailed freight train in Somerset will cause disruption to passenger services for at least 48 hours, Network Rail said.
The train came off the track at East Somerset Junction between Westbury and Castle Cary at 5.50pm on Monday.
There were no injuries but major damage was caused to the track and rail equipment, meaning the line is blocked and likely to remain out of use for "at least the next 48 hours", Network Rail warned on Tuesday morning.
Great Western Railway and South West Trains passengers are advised to check for updates before they travel.
Services between Taunton and Westbury, and between Weymouth and Westbury, are being severely disrupted.
High-speed trains between London Paddington and Penzance are being diverted via Bristol Temple Meads.
Engineers are on site assessing the damage and a further update is expected later on Tuesday.
In November 2008 a freight train derailed on a set of points at the same junction, causing damage to the track.
The Rail Accident Investigation Branch found the cause was a signaller, who was "probably suffering from fatigue", not operating the points correctly.