The RAIB (Rail Accident Investigation Branch) has published a report into the derailment of freight wagons at a busy junction in south London. https://www.gov.uk/government/news/report-042018-freight-train-derailment-at-lewisham. The derailment occurred on recently re-laid track which was formed out of pre-fabricated modular units. The cause of the derailment was a “track twist” which occurred because of the poor support given to the track bearers by the underlying formation and the flexibility of the joins between modules.
The Chief Inspector of Rail Accidents commented “The designers of the new layout had not fully understood how this type of assembly could behave if the track bearers were not fully supported by the ballast”. He added “RAIB has investigated the interaction between poor track geometry and unevenly loaded trains several times before and has recommended action to deal with the problem. It is of concern that, although the railway industry has established a working group to examine these issues, it remains unclear how its findings will be translated into actions to mitigate the risk of freight train derailment”.