A jogger forced a train driver to apply the emergency brakes after he found her running in the middle of the tracks.
The woman jumped to the side of the tracks as the train came to a halt near Bucknell railway station in Shropshire.
The driver of the Shrewsbury to Swansea service, who rounded a curve before seeing her, was left in shock and unable to continue driving on Monday.
Police said the woman held up a hand to apologise and told the train crew she had lost her way on a footpath.
British Transport Police (BTP) officers who were called to the scene could not find the jogger following the incident, which happened at shortly after 1500 BST.
This was a very serious incident which has left a train driver in shock and the consequences could have been far worse had the driver not acted so quickly
British Transport Police
The train was travelling at about 60mph (97km/h) when the driver saw her running with her back to the train.
There was major disruption to services after the train driver was unable to carry on, police said.
A BTP spokesman said: "This was a very serious incident which has left a train driver in shock and the consequences could have been far worse had the driver not acted so quickly.
"I cannot emphasise the dangers of trespassing on the railway strongly enough as, unlike road vehicles, trains are unable to stop quickly in an emergency."
The woman police are trying to trace is described as being aged about 40, 5ft 6ins and wearing a blue vest top with white trim and blue shorts.
Anyone with information is asked to contact BTP or Crimestoppers anonymously.