Apart from early press reports and some industry specific blogs about the fact that an RAIB had been instigated there has been no official acknowledgement on the GOV.UK site.
Process dictates that Network Rail, Southeastern Trains and BTP have to report a de-training incident of this gravity.
RAIB then decide what action to take.
They come pre-prepared with powers to enter all properties and seize all records related to the incident(s).
In another authorised de-training incident in London, where the supervising staff realised the 3rd rail had not been fully isolated before the evac had commenced and passengers were already on the track, in November 2017, the RAIB preliminary report was published only weeks later in December 2017.
These incidents are treated very seriously.