SPEED restrictions remained in place last night on a section of the East Coast Main Line where a culvert collapsed.

Railtrack said it was continuing investigations into the fault just north of the Croxdale Viaduct, near Durham.

Engineers are anxious to discover what caused the culvert to collapse, and tests are being carried out to ensure the problem has not weakened the track bed above.

The culvert collapse was discovered by workers on Tuesday morning. Railtrack put a speed restriction on the track section immediately and closed the northbound track temporarily.

The prompt action led to severe delays as trains stacked up on the East Coast Main Line while engineers worked on the culvert.

Officials said a section of the track had to be closed for repair work to go ahead.

Five mph speed restrictions were put in place on a 22-yard section.

The speed limit was raised to 20 mph yesterday.

Passengers had to sit and watch as the Intercity 225 expresses - which usually travel at more than 100 mph between Newcastle and London - slowed to a crawl.

A Railtrack spokeswoman said engineers had been able to pack the culvert with ballast to carry the weight of the trains passing over it.

She said speed restrictions would remain in place until further notice. In the meantime, staff from Railtrack are continuing to monitor the track bed to make sure services are not at risk.

Train operator GNER said the restrictions were adding about ten minutes to scheduled journeys, but reported no major problems.

Passengers have had to put up with a summer of delays on the Edinburgh-London line this year.

In June, the line was closed when a lorry became wedged under a railway bridge - three weeks after concerns were raised about its safety. Services were delayed while safety experts checked the bridge in Albert Hill, Darlington.

Last week, services were disrupted when storms caused flooding between Darlington and Northallerton.

The line has been beset byserious problems even further back, delaying trains to the point where letters from GNER chief executive Christopher Garnett were left on train seats last December apologising for "really rather poor" standards.

Among the reasons cited were that two trains lost in the Hatfield and Selby rail crashes had still not been replaced.