The Rail Accident Investigation Branch (RAIB) has released its report into the derailment of a Class 175 train at Oubeck North near Lancaster on 4 November 2005.
The full report is available here

On Friday 4 November 2005 a train travelling on the West Coast Main Line derailed after running into a landslip at a cutting at Oubeck North. The trailing wheelset of the leading bogie derailed and the train travelled a further 1430m before coming to rest in an upright position. There were no injuries as a result of this derailment but track damage was sustained.

The immediate cause of the derailment was the train running into material deposited on the track as a result of the cutting landslip. This failed due to the volume of water flowing through a concealed field drain into the slope, which was greater than usual due to the wet period over the preceding two weeks and  excessive rain during the previous day. The root cause was that the field drain was hidden from view and therefore the infrastructure owner was unaware of it.
Recommendations

The RAIB has made six recommendations as a result of its investigation. The primary focus of the recommendations was on modifying existing drainage arrangements at Oubeck and identifying, prioritising and managing other cutting slopes prone to risks of earth flow due to drainage from neighbouring property.