A track worker removing cables from a rail line dodged an approaching passenger train less than two seconds before it passed at 51mph.
The worker had been taking away short-circuiting straps, which had been attached to rails near Gatwick Airport station, for railway engineering work.
He did not suffer any injuries during the event, which happened at 11.25pm on 2 December.
An investigation into the circumstances of the incident was announced today by the Rail Accident Investigation Branch (RAIB).
Investigators are examining what happened, including the actions of the track worker and other staff, as well as the rules and procedures applicable to the type of job.
Last week it was revealed that two workers had to jump out of the way of a train travelling at 101mph in Bedfordshire on 12 December.
Neither worker was injured, the RAIB confirmed.