The continuing investigation will now consider elements including how that section of track was inspected and maintained, the condition and maintenance of the embankment, the weather conditions and how effective any hot weather mitigation measures were.
In a joint statement, Lucy McAuliffe, Network Rail’s route director for Sussex, and John Whitehurst, chief executive of Greater Thameslink Railway, apologised for the derailment.
They added: “We will carefully consider any recommendations arising from the investigation and work across the industry to implement them, with the shared goal of making an already safe railway even safer for our customers, colleagues and the communities we serve.”