...the train "jumped its air and hand brakes" shortly before it was officially due to leave Caterham.
Other trains were held back to give the runaway a clear run and it was diverted at Norwood Junctiom.
Prompt action by the signalman at Caterham prevented any serious accident. Under an emergency bell system all level crossing points were aware of the runaway, and early morning traffic was quickly halted.
Southern Region services were not seriously disrupted, and British Rail said they were back to normal soon afterwards.
The fail-safe device known as the dead man's handle, which prevents a train from continuing along the track when a driver has been taken ill, would not have affected the train's journey as the device works only when the power has been turned on.
A spokesman for Southern Region who described the incident as a "a controlled runaway movement" said: "Nothing like this has ever happened before. We are mystified as to how the train shed its brakes.
"The route from Caterham is all downhill, so the train obviously gathered momentum, but it was prevented from reaching any grate speed by its electric motor which is situated on the axle..."
