Somerset freight train derailment causes '48 hour blockage'
A freight train carrying stone and ballast which partially derailed will cause disruption for at least 48 hours.
The train is blocking the line between Castle Cary, Somerset, and Westbury, Wiltshire, following the incident at 17:50 GMT, Network Rail said.
No-one was injured but there was significant damage to track and equipment.
Services from Taunton to Westbury and between Weymouth and Westbury will be severely disrupted.
GWR said trains will be amended or cancelled, with replacement road transport provided where required.
The freight train was pulling 38 wagons, when seven of them partially derailed.
A Network Rail spokesman added: "We are working closely with GWR and South West Trains to put in place contingency plans... including diverting London-West of England services via Bristol.
"No injuries have been sustained but there is significant damage to track and equipment.
"The derailment is significant with the line currently blocked and likely to remain out of use for at least the next 48 hours."