The Rail Accident Investigation Branch (RAIB) has released its report into a collision at Badminton on 31 October 2006.
The full report is available here:

A summary of the of the report is: On Tuesday 31 October 2006 two self-propelled track maintenance machines, a tamper and a ballast regulator, collided near the site of the former station at Badminton, Gloucestershire. The collision occurred on a section of the line that was closed to normal traffic for track renewal work. The tamper was travelling at approx 35mpn and the ballast regulator was stationary. All four members of staff on board the machines were injured, two of them seriously.

The immediate cause of the accident was the driver of the tamper not controlling his speed or reacting to the presence of the stationary regulator on the line ahead. Causal factors were the fatigue experienced by the driver, and the running of the tamper and regulator separately, rather than coupling them together. Contributory factors were the custom of maintenance machines being driven at more than 20mph in work sites, and the lack of a defined speed limit for these movements.