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25 January 2016, 19:04 | Updated: 25 January 2016, 19:06
A probe is to be carried out after trains passed over a viaduct following storm damage, rail investigators have confirmed.
The Lamington Viaduct in South Lanarkshire, a section of the West Coast Main Line between Glasgow and London, is expected to remain closed until March for repairs.
Damage to the structure, which carries the railway running between Carstairs and Lockerbie across the River Clyde, was discovered following high water levels brought about by Storm Frank at the end of last month.
The Rail Accident Investigation Branch (RAIB) said several trains passed over the crossing after the ground beneath a viaduct pier had been eroded by river scour on December 31.
It said the southbound track was inspected and reopened after a potential problem was reported but a fault was soon spotted after a northbound train passed at high speed.
There were no injuries or damage to trains but investigators described the viaduct as ``seriously damaged''.
They said a driver reported a dip in the track at around 7.35am on Hogmanay and signallers restricted the speed of trains.
Track maintenance staff watched a train pass at low speed and removed the speed restriction while they remained on site to undertake minor remedial work, the RAIB found.
But restrictions were then re-imposed in both directions after they observed ``unusual track movement'' at 8.40am when a train passed at high speed, investigators said.
They then blocked the line to all trains a few minutes later after spotting a ``large crack'' in one of the piers.
A subsequent investigation has revealed a large hole beneath the pier and damage to three of the steel bearings which support the bridge deck, according to the RAIB.
After the closure of the viaduct on January 10, a disused part of another pier collapsed.
The RAIB's findings, including any recommendations to improve safety, will be published at the end of the investigation.
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.''