A London Overground train hit a piece of metal on the tracks at West Croydon station, causing severe delays on the network.
Services on the southbound line came to a standstill after the 10.12am from Highbury and Islington hit the object.
The soutbound line from the station was closed as a "precaution" while the track was inspected for damage.
A spokeswoman from Network Rail said there was no damage to the train or the tracks and the line has now reopened.
Delays between West Croydon and New Cross Gate are expected until at least 3pm.
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