Train allowed to cross damaged Lamington Viaduct
A train was allowed to cross the Lamington Viaduct at high speed after the crossing suffered serious storm damage, it has emerged.
The Rail Accident Investigation Branch (RAIB) said it would carry out an investigation into what it described as a "dangerous occurrence".
The viaduct, which carries the West Coast Main Line over the River Clyde, was damaged by the swollen river on Hogmanay.
It remains closed for emergency repair.
The incident being investigated by the RAIB took place on the day the damage occurred, before a decision was made to close the line.
Investigators said the driver of a Virgin West Coast service from Edinburgh Waverley to London Euston reported a potential problem at 07:35 on 31 December.
He said there was a dip in the track. The area had experienced high levels of rain because of Storm Frank.
In response, signallers restricted the speed of trains on the section of the line affected.
Maintenance staff inspected the track and watched a southbound train pass over it at low speed.
They then decided it was safe to lift the speed restriction.
But when a northbound train crossed the viaduct at high speed at 08:40, they noticed what was described as "unusual track movement".
That prompted them to immediately re-impose the speed restriction.
A few minutes later, at 08:57, they closed the line to all trains after finding a large crack in one of the piers supporting the viaduct.
A more detailed investigation has since found a large hole beneath the pier, along with subsidence and damage to three of the steel bearings which support the bridge deck.
After the closure of the line on 10 January, a disused part of another pier under the viaduct collapsed.
Bridges over rivers can be liable to "scour damage", where currents in the water undermine the structure.
The RAIB said it will look at:
- Actions taken in response to a scour risk assessment undertaken for Lamington Viaduct in 2005
- Actions taken when the approach of Storm Frank resulted in warnings of exceptionally heavy rain
- Responses to the report on 31 December of a track dip on the viaduct
- The effectiveness of Network Rail's processes intended to mitigate the risks to structures... due to extreme weather
- Any underlying management factors
A Network Rail spokesman said: "The safety of passengers and our workforce is of vital importance and we are already conducting our own internal review of the incident at Lamington.
"We will work closely with the RAIB as it completes its inquiry."
Engineers said last week that they expect the West Coast Main Line to remain closed at Lamington Viaduct until the beginning of March.