At about 05:30 hrs on 24 January 2017 two wagons within an aggregate train derailed on newly-laid track at Courthill Loop South Junction in Lewisham, south-east London. The first of the wagons ran derailed, damaging the track, then overturned spilling its payload of sand. There was major disruption to rail services while the wagons were recovered and the infrastructure reinstated. No-one was injured.
The new track had been installed during renewal work on the weekend of 14 and 15 January 2017. It was made up of separate panels of switch and crossing track, comprising the rails, point and crossing components and the supporting concrete bearers. Most of these track panels had been brought to site pre-assembled. A mechanical connector, known as a ‘bearer tie’, was used to join the concrete bearers that were designed to support rails on more than one panel. Network Rail originally developed the concept for this type of track in the mid-2000s; it is referred to as ‘modular S&C’.
Planned follow-up engineering work was undertaken on the subsequent weekend. The derailment happened on the day after hand-back checks on completion of this work had confirmed that the track geometry was suitable for the passage of trains. It occurred because the first of the two derailed wagons, which was probably carrying an uneven payload, encountered a significant track twist, resulting in there being insufficient wheel load at the leading left-hand wheel to prevent its flange climbing over the rail head.
The track twist had developed rapidly following the hand-back because:
- the support offered by the track bed to the concrete bearers was poor
- the inherent flexibility of the bearer ties located between the two running rails made one side of the track more susceptible to the poor track bed support than the other.
Network Rail’s engineering processes for specifying and developing modular S&C layouts were an underlying factor, in that they were inadequate for controlling the risks associated with flange climb derailment.
The RAIB has made five recommendations:
- Four are directed to Network Rail, concerned with:
- the processes it uses to identify and manage risks associated with vertical track geometry features following track renewal and heavy maintenance
- the design and validation of its modular S&C layouts
- One is directed to RSSB concerned with understanding and managing the derailment risks associated with uneven loading of bulk hopper wagons
The RAIB has additionally identified learning points concerning the management and planning of track installation work, and procedures for the routine maintenance of railway vehicles.
Published 28 February 2018