British Transport Police said that officers and other emergency services were responding to the derailment of a passenger train near Lewes railway station in southeast England on Thursday.
Pictures of the scene on social media appeared to show at least one carriage lying on its side after the incident about 45 miles (72 km) south of London.
There were no immediate reports of any casulaties.
Transport minister Heidi Alexander said in a post on X that she was deeply concerned by the incident and said the government was working quickly with the rail industry to assist passengers.
