The Rail Accident Investigation Branch (RAIB) has said it is probing the July 1 incident, which has been described as a “near miss”.
According to the watchdog, the access ramp had been attached to the doorway by platform staff to help a passenger with a mobility scooter who wanted to board the train.
However, the driver was reportedly unaware that the access ramp was attached when they closed the doors, and the train departed with it still attached.
No one was on the ramp as the train set off, but two members of staff and a passenger on the platform “took action to avoid” the access ramp as it was dragged along.
After travelling for 160 metres, the ramp struck a fence at the end of the platform and fell away from the train.
The train was eventually taken out of service at New Cross Gate. No injuries were reported, although damage was caused to the access ramp.
Services on the Windrush line are operated by Arriva Rail London on behalf of Transport for London (TfL).
The RAIB has said its investigation will consider a number of factors including the actions of those involved and anything that may have influenced them.
Their probe will also look at the competency and training of the staff involved, and the arrangements in place to manage the risks associated with manual access ramps.
A TfL spokesperson, said: “We take safety extremely seriously and we are thankful that no one was injured during an incident which occurred as the train departed.
“We welcome the investigation by both Arriva Rail London, our operator, and the RAIB to fully understand what happened and to implement any necessary changes to ensure this cannot happen again.”
