It's clear and straightforward. Section 11.2 of the Railways (Accident Investigation and Reporting) Regulations 2005 requires RAIB (Read About It Belatedly) to "publish a final report [into any event it investigates] in the shortest time possible and normally not later than twelve months after the date of the occurrence".

(Click here to read the full regulations.)

The word 'normally' is not a catch-all get-out clause. The generous deadline is 12 months unless - according to the safety unit of the European Railway Agency - there are "justified reasons" why it cannot be met.

The graph and list below analyses 'heavy rail' accidents or incidents into which RAIB has conducted inquiries. As you can see, compliance with regulation 11.2 is far from 'normal', compromising the ability of the wider industry to learn lessons quickly and effectively from these serious events.

The data used to produce the above graph only considers RAIB's 'heavy rail' inquiries - those relating to events on Network Rail infrastructure.

Since 2008, the average duration for the 190 heavy rail inquiries completed by RAIB is 361 days. Of these, 64 have taken longer than the regulatory deadline, or 34% of them.

RAIB's performance was characteristically poor in 2014 and through the first half of 2015, but then improved markedly. The Branch missed the stated 12-month deadline on 11 occasions in the 18 months to June 2015, but its incident-to-publication average for the 12 reports released thereafter was 303 days: well short of the six-month timescale considered to the "reasonable" by senior railway managers, but much better than its previous performance. Which begs the question, why have things been so bad for the previous ten years? Of course, time will tell whether this improvement is maintained.