An empty passenger train had to stop when the driver spotted another train on the same track outside Brighton station today.
The incident comes exactly a year after the Paddington rail disaster.
Train operators Connex said the incident at 9am was caused by a Railtrack signal operator failing to tell the passenger train driver that another train was up ahead.
She said the passenger train came to a "gentle stop" and the incident has not been classified as a near-miss.
The spokeswoman said: "It was what we call a legal move, but an incident report has been logged and we have spoken to staff about it."
The three-coach SE33 was leaving platform one to form a longer train and had moved the distance of three coaches when the driver saw the tail lights of a two-car snow remover train some distance ahead.
The Connex spokeswoman said: "Our driver did not know the other coaches were on the track but he stopped with plenty of room to spare.
"It shows how good our drivers are and how well our procedures work."
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