SPEED restrictions remain in place today on a section of the East Coast Mainline where a culvert unexpectedly 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 hasn't weakened the track bed above.

The collapsed culvert was discovered by workmen checking the line on Tuesday morning.

Railtrack immediately slapped a speed restriction on the track section and temporarily closed the northbound track.

The prompt action led to severe delays as trains stacked up on the East Coast Mainline while engineers battled to put the collapse right.

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 of the track which was today raised to 20 mph.

Bemused 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.

The spokeswoman 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 aren't 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.

And only last week services were disrupted when torrential rain storms caused flooding between Darlington and Northallerton.