A train on the Norwich to Liverpool Street line has come to a standstill at Witham after a passenger was taken ill on board.
The 11am train stopped at about 12.40 after the passenger became ill.
An ambulance is now at the scene at Witham station, with mainline trains being diverted around the blockage.
A spokesman for the East of England Ambulance said: "Two ambulances and paramedic car plus the air ambulance went to the scene.
"The casualty is on their way to Broomfield Hospital at the moment by land ambulance and one of our critical care team are travelling with them treating the patient."
The nature of the person's condition is not known yet.
Passengers can expect delays of up to five minutes on the London-bound line whilst the service recovers.
The 11:30am Norwich to Liverpool Street and the 12:05am Clacton-On-Sea to Liverpool Street trains are delayed due to this incident.
