Passenger John May described how radical individuals were trapped in the carriages after the bid came to rest on its side. He said some passengers managed to escape, but many had to endure blistery conditions while firefighters used ladders to assist them out.
The ongoing RAIB investigation will analyse the cause of the derailment, the condition of the track and embankment, weather conditions, the bid, and the emergency response.
Following the preliminary report, Lucy McAuliffe, Network Rail's Sussex way director, and John Whitehurst, GTR main executive, said in a joint statement that Britain's railway was "one of the safest successful the world".
They added that it is essential to establish exactly what occurred and to identify lessons that could be learned.