A Chiltern Railways train narrowly avoided a head-on crash after travelling the wrong way up a London Underground track.
The train stopped just two metres in front of a Metropolitan line Tube at Chalfont and Latimer, the junction for the Chesham branch.
No one is believed to have been hurt after the incident, around 9.50pm on Sunday, June 21.
Frustrated passengers were forced to find alternative routes home after the Metropolitan line was suspended between Rickmansworth and Chesham.
Inspectors from the Rail Accident Investigation Branch (RAIB) are now looking into the incident, which is said to have caused damage to parts of the track.
A formal probe may be launched if it is decided there are “important lessons” for the railway industry.
An RAIB spokesperson said: “The Rail Accident Investigation Branch was notified of near-miss between two trains at Chalfont and Latimer.
“Our inspectors are gathering evidence as part of the process of conducting a preliminary examination.
“If the evidence collected indicates that there are likely to be important safety lessons for the railway industry then a full and thorough investigation will be carried out and the findings, along with any safety recommendations, published.
“The decision will be made in the next few days.”