The Rail Accident Investigation Branch (RAIB) has released its report into two derailments at London Waterloo, on 11 September and 24 October 2006.
The full report is available here:

A summary of the key points from the report are: On Monday 11 September an empty South West Trains service was moving into Waterloo south sidings at low speed when it derailed on a set of points. These points had recently been subjected to unplanned maintenance.

On Tuesday 24 October a South West Trains service from Dorking to Waterloo derailed on a different set of points on the approaches to Waterloo. These points had also recently been subject to unplanned maintenance. Again, the derailment happened at low speed and there were no injuries. Some signalling and electrification equipment was damaged in the incident, and services from Waterloo were affected for several days while repairs were undertaken to the affected points and others exhibiting similar defects.

The immediate cause of both derailments was the condition of the switch blades within each set of points. These exhibited known derailment risks which had not been identified by the routine inspection process or by detailed inspections following maintenance activity. There were four causal and 13 contributory factors to the derailments.

There were six underlying causes to the derailment, which covered records management, understanding of standards and their requirements and inadequate track access for checking points.