A Southern Railway commuter train without a conductor travelled a short distance with its doors open, the operator said.
The incident occurred at Reedham station in Purley, Surrey on Monday shortly after 6am.
Southern has been embroiled in a bitter dispute with the Rail, Maritime and Transport union over the role of conductors.
The operator is introducing a new on-board superviser role which focuses on customer service and moves the operation of closing doors to the driver.
A spokesman for Southern said: "Yesterday morning there was an operating incident at Reedham station involving the 0554 service from Tattenham Corner to London Bridge.
"There was nothing wrong with the train and early indications are this was driver error in releasing the brakes before the doors had shut.
"This allowed the train to roll downhill unpowered one coach length at low speed before the driver realised his error.
"This was an unusually rare set of circumstances on a train that has run without a conductor for many years and could have happened even if there had been a guard on board."
The Rail Accident Investigation Branch said it was aware of the incident but was not carrying out an investigation.
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