https://www.gov.uk/raib-reports/report- ... =immediate
At around 21:30 hrs on 6 May 2019 a single wagon in a freight train derailed on a curve approaching Willesden High Level Junction in north-west London. The wagon re-railed as it passed over the junction. Although no-one was injured, a derailment like this has the potential to foul lines that are open to passenger traffic or strike structures.
The track was supported by an earth embankment that Network Rail had been monitoring since October 2016, and which was showing signs of progressive seasonal movement. The empty two-axle wagon derailed where the track cross-level had been changing. Derailment occurred because the wagon encountered a significant track twist and had an uneven wheel load distribution.This combination resulted in there being insufficient load at the leading left-hand wheel to prevent the wheel flange climbing over the railhead. A check rail would have prevented the derailment. Network Rail had completed a risk assessment that had concluded this safeguard was not necessary on the small-radius curve.
My italics
