17-metre bogie spacing key
Coupler energy absorption too stiff
Excessive coupler freedom caused derailment
In the January 2021 column I analysed the human factors in the low-speed collision between a Class 800 and the IC125 it was following entering Neville Hill depot. The IC125 was running at about 5mph and the Class 800 at 15mph.
Despite this low closing speed, the trailing bogies of the second, third and fourth vehicles of the Class 800 derailed to the right, the severity of lateral displacement decreasing along the train (Figure 3). Moreover, the first two vehicles intruded on the ‘six foot’ to the extent that they would have been hit by a passing train on the adjacent track. The passing clearance between two Class 800 units is 665mm, compared with the derailed coaches’ 1-1.25 metre displacement.
Post-accident testing of the Class 800 and the track failed to find any defects which could explain this large displacement from such a low-speed collision. The Rail Accident Investigation Branch’s (RAIB) next step was to review similar accidents over the last 30 years. None of these had resulted in derailment. The most violent, in terms of energy dissipated, was the collision between a Class 87 with 10 Mk 3 coaches and a four-car Class 142 at Winsford in 1999. While the locomotive derailed, the Mk 3 coaches remained in line.