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.

The average duration for all 77 of the above inquiries is 370 days. Of these, 29 took longer than the regulatory deadline.

The target duration is not 12 months - this is the upper time limit. The aim is for inquiries to be published in "the shortest time possible". Senior rail managers, surveyed by Ipsos Mori, considered six months to be a 'reasonable' timescale.

RAIB's performance improved during 2010, largely because it only launched 13 heavy rail inquiries during the whole of 2009 - that's one for every two members of its investigative team. Five other inquiries were also started.