Safety checks were carried out within 24 hours

Royal Mail said the closure of the northern stretch of line, and of another section between Milton Keynes and Rugby, had seriously affected its operation.

Railtrack came in for severe criticism over its handling of the closure, which was announced on Tuesday evening.

Train operators ScotRail and Virgin said they had received scant notice and passengers were left stranded at stations where they complained of receiving little or no information.

Railtrack insisted that the closure, which was prompted by last week's fatal derailment at Hatfield, in Hertfordshire, was the "least unpalatable" option.

It insisted that the alternative would be weeks of delays on services caused by speed restrictions.

Tests completed

The company initially warned that the line would be closed until Saturday morning.

However, on Wednesday afternoon Ms Anderson said tests had been completed and the line would reopen much sooner, albeit with curbs on speed.

She said: "We have detected 20 locations which require some remedial work, including rerailing, which we will be doing overnight and we're going to reopen the line with some temporary speed restrictions.

Janette Anderson said closure was the only option

"So I should take this opportunity to let people know that there are going to be some delays tomorrow (Thursday) but it is important to know that safety is our number one priority."

Despite the move, ScotRail said it was still reviewing sleeper services and no new bookings were being taken.

Royal Mail said its operations had also been affected by "unacceptable delay".

A spokesman said: "The closure without warning of the West Coast Main Line in Scotland, and between Milton Keynes and Rugby, has hit our mail carrying trains hard."

He revealed that Royal Mail has switched to using five extra air services and about 20 extra road vehicles to minimise disruption.

ScotRail said passengers seeking more information should contact National Rail Enquiries on 08457 484950, or access the ScotRail website on www.scotrail.co.uk.