A police helicopter was called to Witham to search for a man who allegedly exposed himself on a train.
The chopper scoured the area near Witham Railway Station after the alleged incident, which happened at about 3.30pm last Saturday on a London Liverpool Street to Witham service train.
A British Transport Police (BTP) spokesman said: “The victim was a woman in her 30s who joined the train at Stratford.
“She was sitting in the front coach when she noticed a man walking through the train.
“He stood by the doors at the front of the carriage, as the train passed Shenfield, and then exposed himself before leaving the train at Witham.”
The man is described as white, 5 ft 10” tall, of medium build and in his mid to late 30s, with short, brown hair.
He was wearing light blue denim jeans, a grey polo short and carrying a black rucksack.
Anyone with information is urged to call British Transport Police on 0800 405040 or Crimestoppers anonymously on 0800 555 111.