An inquiry has been launched after a commuter train skidded on slippery track through two red lights before stopping 300 yards from another train.
The train managed to stop just 300 yards from another train
The train drivers' union Aslef said last Friday's incident, between Esher and Surbiton in Surrey, had the potential to be a "major disaster".
The driver was praised for managing to stop the train, which was travelling at 85mph, and alert a signalman.
An investigation will probe possible rail contamination or unit failure.
Driver error has been ruled out following analysis of the black box recorder on the train.
Network Rail said the incident was being investigated to establish exactly what happened.
It is believed that there were no objects or leaves on the line and the brakes of the train were working normally.
The weather at the time was wet, cold and foggy.
Graham Morris, from Aslef, said a meeting would be held on 8 December between the union, South West Trains and Southern Trains to review the investigation.
Aslef has stepped up its call for improvements in cab-to-cab communications systems so that drivers can contact each other to warn of potential problems.