An investigation began today into how part of a bridge fell on to the main railway line into Liverpool Street station.

Tens of thousands of commuters faced chaos at rush hour this morning as every train into the station was cancelled. They have been warned to expect more delays on the way home.

Passengers were seconds from disaster when debris from the bridge collapsed on to the track last night.

Only the quick reactions of a driver prevented a packed commuter train travelling at 40mph ploughing into steel and concrete. More than 1,000 passengers on stranded trains then had to walk along the track to Liverpool Street and Bethnal Green stations.

Today questions were being asked of both Network Rail and Transport for London, which installed the bridge earlier this month as part of the East London Line extension. Both organisations faced calls for a public inquiry today.

The investigation is to centre on how the bridge was put in place by workers for Balfour Beatty Carillion, who are building the new East London Line.

It was rolled into position earlier this month over the early May Bank Holiday weekend but workers were still completing the installation last night as trains passed underneath.

Engineers believe a jack holding the bridge in place collapsed, throwing concrete on to the main line out of Liverpool Street. The driver of the 7.15pm service to Southend Victoria rounded a bend and saw the tracks blocked.

Despite slamming on the brakes, the train slid into the debris but the driver managed to keep the train on the tracks. There were no injuries.

But Keith Norman, leader of Aslef, said: "This could so easily have been a major disaster. The driver did well to bring his train to an immediate controlled stop - we wholeheartedly back the calls for a full inquiry into exactly what happened."

Network Rail was forced to turn off the overhead 25,000-volt powerlines, leaving a second train stranded between stations. Those onboard the two stranded trains were led to safety by police and rail staff

TfL and Network Rail worked through the night to repair the line but this afternoon only a partly restored service was being predicted for the rush hour.

Val Shawcross, chairwoman of the London Assembly transport committee said: "There should be a health and safety inquiry into what happened, but these usually proceed quite slowly, so there should also be a fast review of how these engineering works are being conducted to make sure this doesn't happen again."

Demolition work in preparation for the new bridge had already caused disruption to workers returning to work after New Year when it overran at Christmas.

Network Rail was fined a total of £14.4 million and heavily criticised in an inquiry into work overruns at Liverpool Street and on the West Coast Main Line.

But TfL had gone to great lengths to make sure it was not blamed for the overrun of work on the bridge, emphasising that the demolition and the subsequent May installation of the bridge had gone to plan.

A spokesman for Network Rail said: "We have had our engineers there all night along with three of our directors, Simon Kirby, director of infrastructure, Robin Gisby, operations, and Patrick Hallgate, route director. The bridge was declared safe at 7am." TfL confirmed the debris was reported after work by contractors.

SERVICES FROM LIVERPOOL ST TONIGHT

* All services to and from Liverpool Street will be delayed by at least 40 minutes.

* Last-minute alterations and cancellations are possible to all services.

* Direct trains to and from Walton-on-the-Naze are cancelled - change at Colchester; change at Manningtree for Harwich branch services which will not run into Liverpool Street. The Braintree branch has no Liverpool Street services, change at Witham.

* Tube, c2c and First Capital Connect are all taking National Express East Anglia tickets on their routes.

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