Gauge spreading caused the derailment of the Virgin Trains London to Glasgow service at Mossend last Sunday. The rails had spread in excess of 60mm over the normal measurement of 1,435mm, causing the wheels to drop between them.
The section of track is on a tight curve and had a speed restriction of 15mph. Track spread can be caused by defective sleepers, rail fastenings, base plates and certain features of the train.
Railtrack said that track spread cannot always be identified through routine visual inspections, but depend on specialist track recording units.
The track was last visually inspected on 22 November. The sleepers at the site of the accident are timber.