A PASSENGER train was allowed to cross a severely damaged bridge at high-speed after inspectors initially found "no defects" in the structure, an investigation has found.

A probe by the Rail Accident Investigation Branch (RAIB) said that the passage of a Virgin Trains service en route to Glasgow at some 110mph on Hogmanay last year is likely to have contributed to the subsidence of one of the piers supporting the Lamington Viaduct.

The bridge, which crosses the River Clyde near Abington in South Lanarkshire, was subsequently closed for seven weeks until late February while engineers carried out intensive repairs to save it from collapse.

Its foundations had been badly damaged in the days previously as a result of flooding caused by Storm Frank.

The RAIB's investigation into the "dangerous occurrence" is still ongoing, but in their latest update the safety watchdog described how maintenance staff observed "unusual movements" between the carriages of a Crewe-Glasgow Central passenger train five minutes after lifting speed restrictions over the bridge because they could not find any faults with it despite an earlier train driver raising the alarm over an apparent a dip in the track.

The RAIB update states: "At 8:40am, the maintenance staff saw unusual movements between carriages of the 5.57am Virgin West Coast service from Crewe to Glasgow Central as it crossed the viaduct on the northbound line at about 110 mph, and then observed that the passage of this train had caused 'sweeping dips in the track' affecting both lines across the viaduct."

Maintenance staff had been called to the scene on the morning of December 31 2015 after the driver of another Virgin West Coast service, the 6.52am train from Edinburgh Waverley to London Euston, reported a dip in the track as he crossed the Lamington Viaduct at 7.35am.

Signallers instructed subsequent trains to cross the bridge at a reduced speed until it could be inspected.

However, the speed restriction was lifted an hour later, at 8.35am, after the track maintenance staff "found no significant defects" on the viaduct or track.

It was re-imposed just five minutes later after the inspectors observed the "unusual movements" of the 8.40am Virgin train, but a number of passenger trains were still permitted to cross the viaduct at low speed.

The last to cross was First TransPennine Express service en route from Edinburgh to Manchester Airport.

The bridge was finally closed 13 minutes later, at 8.53am, after the maintenance staff discovered "missing masonry and a large crack in pier number two" - one of three support beams holding up the viaduct.

The RAIB report adds: "Subsequent investigation has revealed a large hole beneath pier number two, subsidence of the pier and damage to steel bearings which support the bridge deck.

"The damage was discovered following exceptionally high water levels, and fast flows in the River Clyde due to Storm Frank, which impacted the UK on 29-30 December 2015.

"Evidence found to date suggests that the sweeping dips in the track were caused by deck movement related to the subsidence of pier two, which probably occurred under the last northbound train at 08:40 hrs.

"The related scour damage may have been undiscovered since the river level peaked the previous day."

The investigation continues.