About 500 rail passengers were trapped in a tunnel in south London for 90 minutes when their train separated.
The first four carriages of the 1745 BST Victoria to Rochester train went into Penge tunnel while the final four were left at Sydenham Hill station.
Passengers were trapped in the tunnel until engineers reversed the carriages back into the station. No-one was hurt.
Rail operator Southeastern said a fault in one of the train's coupling units may have been responsible.
The Wednesday evening incident led to many passengers being delayed, the spokesman admitted.
'Frustrating' delay
Passengers on the carriages that remained in the station were let off straight away, while passengers in the tunnel were taken off the train by 1945 BST. The line reopened 45 minutes later.
Southeastern's spokesman said: "While no-one was in immediate danger, we appreciate this would have been concerning for passengers onboard.
"And it would have been frustrating for many as it meant their journeys home were delayed."
Train brakes automatically activate if its carriages divide, as a safety mechanism, the spokesman said.
The train was taken to a depot for inspection as investigations continue.
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