Martin Frobisher, of Network Rail, said the summer’s hot weather had “presented exceptional challenges to the railway”.
The investigation would specifically assess “whether or not this was weather-related”, said Mark Francois, the Conservative MP for Rayleigh and Wickford.
“However, the bottom line is that, thankfully, the passengers appear to be safe,” he added.
The Department for Transport stressed Britain’s railways were “among the safest in the world” as it reacted to the incident.
It said derailments were “incredibly rare” events.
An East of England Ambulance Service spokesperson said: “Our crews assessed 62 patients who were on board the train.
“Following assessment, all patients were discharged at the scene.
“We also assisted with the evacuation of a second train that was unable to continue its journey due to the derailment.
“There were no casualties among those on board.”
A spokesperson for BTP also confirmed there were no reported injuries.
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