The maintenance vehicle was a mobile elevating working platform that was equipped with both rubber wheels for road running and steel rail wheels for operation on the railway.
On the Friday night of the incident earlier this month it was intended to be used for examination work on a section of track which was under possession.
The runaway started at the Britannia Street road-rail access point, which provides a flat surface allowing road-rail vehicles to be manoeuvred on to the track.
This access point is on a section of railway which slopes downwards at a declared gradient of 1 in 50 towards the station before running onto level track as it enters the platforms.
Shortly after 01:30 hrs the MEWP was being transferred from its rubber-tyred road wheels onto its rail wheels.
During this manoeuvre, known as on-tracking, the machine operator was controlling the machine using a remote control unit which was connected to the machine by a cable.
As the rail wheels were lowered onto the track the MEWP started to run down the gradient towards the station where it stopped about 340 metres from the access point.
No-one was on board the MEWP as it started to run away. The machine operator and machine controller attempted to stop the runaway.
They were unable to do so, but they kept up with the machine during its travel to warn other staff working in the vicinity.
Initial evidence suggests that the runaway occurred because the on-tracking was not carried out correctly and the rail wheel brakes, intended to hold the vehicle on the gradient, provided insufficient brake force to do so.
The Rail Accident Investigation Board’s probe will identify:
- the sequence of events that led to the runaway
- the factors influencing the actions of those involved in the operation of the machine as it was being placed onto the track
- the actual capability of the rail wheel brakes
- the rail-conversion and maintenance history of the MEWP
- the standards and design approval processes that were applied to this type of MEWP
- any relevant management factors