Cause of the accident
This derailment was caused because tram 236 was able to call HVP3 points and change their position as the leading wheelset of tram 219 was about to pass over them.
The tram drivers involved in the accident were working according to normal operational procedures and their actions were not a factor in the derailment.
In normal operation, trams travelling north towards HVP3 points from David Lane are detected by a track circuit located on the approach to the points.
Trams are then subsequently detected by a mass detector located at the junction.
This detection should inhibit further operation of the points and ensure that they are not moved while a tram is passing over them.
Once trams clear the track circuit and then the mass detector (in the correct sequence) the points are released to accept a new route for following trams.
Investigations showed that before the accident, the track circuit on approach to HVP3 points had not been operating correctly.
This meant that the points control system was registering the presence of a tram through the mass detector without the track circuit having first detected the tram’s approach.
As this was contrary to the expected sequence, the control system generated an ‘out of sequence’ alarm and prevented the points from moving as a ‘fail-safe’ measure.
The technicians who responded to this failure noted that the lineside points controller unit was displaying fault code ‘FL02’.
However, they were unaware of the meaning of this fault code.
A laminated sheet in the points control cabinet stated that one possible cause of an ‘FL02’ fault is a ‘sequence error’.
Although the technicians were unaware of the nature of the fault, they discovered that they could clear the ‘FL02’ fault code from the points controller by either turning the controller off and on again or by using the buttons within the points controller cabinet to swing the points manually.
To allow trams to continue operating while the technicians were investigating the fault, they adopted an improvised method of work.
This involved clearing the fault code to allow approaching trams to call the points into the position required for their route.
Once the route was called, the technicians then stopped work and waited for northbound trams to depart from David Lane tram stop and to clear the junction before restarting work.
In line with this improvised system of work, the technicians stopped working on the points controller as tram 219 approached from David Lane, in the knowledge that the fault had been cleared and that the tram could continue on the set route.
However, the technicians were unaware that by resetting the controller, the stored route request from (in this case) tram 219 was removed from the points control unit.
This meant that the controller would accept, and act on, a subsequent call (from another tram) for the points to move.
The absence of the protection provided by the track circuit meant that the points could, in such circumstances, swing with a tram approaching them.
In these circumstances, the last opportunity to avoid the derailment would be a tram driver observing that the PPI immediately before the points was showing an incorrect indication and observing the point blades themselves, as required by NET procedures.
However, in the case of this accident, the points moved after both the PPI and the point blades had passed out of the view of the driver of tram 219.
NET explained that there is no written plan for dealing with the ‘FL02 sequence error’ identified on HVP3 points.
However, NET procedure QP/ENG/GEN/0003 requires staff working on or near the tramway to undertake a Worksite Safety Assessment (WSA) where written plans for the work to be carried out do not already exist.
The staff attending the site in response to the fault did not create a WSA, nor was any other plan devised to help understand the risks that their improvised system of work might inadvertently create.
Consequently, they did not adopt suitable mitigations against the risks created by working on the points while trams were running.
