RHTT risks
The Rail Accident Investigation Branch (RAIB) has issued a safety digest following a derailment of a railhead treatment train (RHTT) at Dunkeld and Birnam station on October 29 2018.
Its investigation found that the leading wheelset of the trailing bogie of the leading wagon began to slide rather than rotate, creating a large wheel flat on both wheels and creating ‘false flanges’.
When passing over points south of the station, the ‘false flange’ on the left-hand wheel became trapped between the stock and switch rails. This caused the outside face of the wheel to run along the gauge face of the stock rail while the right-hand wheel was pushed up against the opposite stock rail.
This led to the wheelset pushing the rails apart, resulting in high lateral forces on the stock rails which in turn overloaded the rail fastenings and, in some places, the wooden sleepers. This derailed the leading wheelset of the bogie, shortly followed by the trailing wheelset. They ran derailed for approximately 100 metres before rerailing at another set of points.
RAIB’s investigation found that the handbrake interlock which prevents air brakes from being released on the leading wagon had been isolated - but it is not clear why or by whom. Because the handbrake interlock was isolated, there was nothing to stop the driver driving the RHTT with the handbrake accidentally left on or partially applied.
RAIB says operators and maintainers of RHTTs should closely monitor the condition of wheels and braking systems when operating in low-adhesion conditions.
It adds that operators of freight trains and other specialist trains derived from freight wagons should undertake suitable roll-by examinations on departure from yards to detect non-rotating wheels; that maintainers of freight wagons should have a process in place to control the isolation of handbrake interlocks on freight wagons; and that staff preparing freight trains for departure should check that handbrakes are fully released and do not place sole reliance on handbrake interlocks.