Rail investigators are now probing the incident which forced maintenance teams to scatter as the vehicle careered out-of-control for more than a mile.

The maintenance vehicle was a mobile elevating working platform (MEWP) equipped with both rubber wheels for road running and steel rail wheels for operation on the railway.

The runaway late last month started at the Keppochhill road-rail access point when the MEWP was being transferred from its rubber tyred road wheels onto its rail wheels.

During this manoeuvre the machine operator was controlling it via a remote control unit connected by a length of cable.

As the rail wheels were lowered onto the track the MEWP started to run down the gradient, through the tunnel and into the station where it finally stopped after about one mile.

No-one was onboard the MEWP as it ran away but while passing through the tunnel it struck some scaffolding which was being erected as part of the planned engineering work and a person working on the scaffolding was seriously injured.

Other members of staff working on the track were able to move clear of the runaway vehicle because they either heard its approach or were warned by mobile phone.

A preliminary examination by rail investigators at the RAIB has shown that the runaway occurred because the brakes acting on the rail wheels were inadequate to stop the vehicle on the gradient and the road to rail transfer was not carried out correctly.

The examination also showed that the MEWP ran through the tunnel without lights.