From where the train was left in platform 3 to where it stopped, Network Rail data sources record there is an average falling gradient of 1 in 348 (0.29%).
RAIB calculated that if the train had no applied brake force at all, it would have rolled away on falling gradients for over 0.6 miles (1 km), at speeds of up to 11 mph (18 km/h), until it reached a 1 in 70 (2.86%) rising gradient.
The train would then have come to a stop and rolled back towards the station, before running back and forth several times until it settled in a dip about 800 metres from the station.
Track gradients in the area
RAIB calculated that with more than 9.5 kilonewtons (kN) of brake force, the train would not move.
With just under 9.5 kN of brake force, the train would roll very slowly and stop soon after the gradient changed to a shallower falling gradient at about 450 metres from the station.
When the train ran away, the signalling system logged when track sections were occupied or became clear during its movement.
Using this data, RAIB calculated that the train moved at an average speed of about 2.2 mph (3.6 km/h) as it rolled out of platform 3.
This slow speed suggests that the train was applying just under 9.5 kN of brake force.
The train travelled about 150 metres, with locomotive 66953 stopping on the set of points that it damaged.
These points were in the opposite position for the train’s movement, so the train ran through them in the trailing direction.
It is likely that when the leading wheels of locomotive 66953 forced their way through the closed switch rail at the points, the additional resistance caused the train to stop, so it did not run away further, exit the area under possession and enter open lines where trains were running.
The parking brake on a class 66 locomotive is required to hold the locomotive on a gradient of 1 in 30 (3.33%) by applying the brake blocks onto the wheels of one wheelset.
This means that when fully applied, the parking brake would need to provide a minimum of around 42 kN of brake force.
RAIB has calculated it was likely that the parking brake on locomotive 66953 was providing less than 22% of this brake force, indicating that it was only partially applied.
To apply the parking brake on a class 66 locomotive, the driver presses a push-button in the cab.
This causes an electric motor to operate that tightens a chain, which in turn pulls the brake blocks onto the wheels of a wheelset.
The motor runs until it detects that the chain is tight enough (so the brake block force is high enough), which is based on how much current the motor is drawing as it pulls on the chain.
While the chain on locomotive 66953 was found to be tight when checked afterwards, this only provided an indication that the brake blocks were pushing against the wheel treads.
It did not indicate that the parking brake was fully applied.
Full application of the parking brake can only be determined by looking at the indication in the cab which should display “ON”, or by checking its status using the locomotive’s onboard computer.
Both require the locomotive’s batteries to be connected.
Previous runaway incidents involving the parking brake on class 66 locomotives have shown that sufficient time must be allowed for the parking brake motor to run, so that it can pull the chain to the required tightness to fully apply the parking brake.
Instructions issued by another freight operating company require its drivers to wait at least 60 seconds for the parking brake to fully apply.
As the data recorder on locomotive 66953 only recorded data for 12 seconds when its batteries were reinstated by driver B to apply the parking brake, it is probable that the batteries were isolated again before the motor had finished running to fully apply the parking brake.
Rule Book module TW1, ‘Preparation and movement of trains’, includes rules about leaving a locomotive unattended.
In section 36.2, it instructs drivers that each time they leave a locomotive unattended, they must make sure it is properly secured.
After the incident, driver A accepted that he had not left the train properly secured before he left it, as he had not checked the parking brake on locomotive 66953 or applied any of the wagon handbrakes, and he had released the parking brake on locomotive 66955.
Although driver B had told driver A about the status of the wagons’ handbrakes at handover, driver A stated that he forgot that the handbrakes were not applied on the wagons.
Driver A had not followed his usual routine when leaving a train unattended, as both locomotives were already shut down with their batteries isolated.
Normally he would have shut the engine down on the leading locomotive, applied the parking brake on that locomotive, isolated the batteries on it, and then walked alongside the train and applied the handbrakes on at least three wagons.
Instead, driver A had spent a long period of time with very little to do.
He stated that when he came to leave, he also thought that the train was secure because that was how he had left it at the end of his previous shift, and the train had not moved since then.
Well here's what happened when I switched the Christmas lights on.........
