Engineers have been inspecting the damage to railway infrastructure after a cement freight train derailed in Carlisle.
Network Rail and the RAIB are working to find out how the wagons left the track and ended up on the embankment and in the nearby river.
With initial inspections to the damage, as well as recovering the train, will take weeks rather than days.
No one was injured in the incident, but major disruption is now being caused for services between Carlisle, Newcastle, Appleby and Skipton.
Rail replacement buses will run to keep people on the move over the next two weeks, and passengers are advised to check www.nationalrail.co.uk.
Phil James, Network Rail's North West route director, said: “On the rare occasions trains leave tracks like this it can cause extensive damage and unfortunately this incident is no exception.
“I understand this will be extremely frustrating for passengers who rely on this crucial rail link from east to west linking Carlisle and Newcastle, as well as south to Skipton. Across the rail industry we're working hard to keep people on the move through rail replacement buses while we work as fast as we can to restore the railway for passengers and freight.”
Kerry Peters, regional director for Northern, said: “We are working closely with Network Rail to monitor the situation and provide the latest information to our passengers as to how they can still travel across the region. Customers should check before they travel and plan for longer journey times. We will provide updates on our website and via social media as soon as any new information about the recovery operation becomes known to us.”
A spokesperson for the Rail Accident Investigation Branch said: “We have launched an investigation into the freight train derailment which occurred at Petteril Bridge Junction, Carlisle yesterday evening (19th October). We currently have a team of five inspectors plus support staff on site. This team is gathering evidence and we will maintain a presence at the site as necessary, over the coming days.
“We are working in conjunction with other agencies to secure the vital evidence needed to help us understand what went wrong. Our inspectors are in the process of reviewing digital material from on-train data recorders, CCTV and infrastructure monitoring systems, as well as collecting physical evidence from the train and track involved.
“In accordance with our normal processes we have started releasing parts of the train. We will continue to release parts of the train and track in stages at the earliest opportunity, consistent with gathering sufficient evidence. A further update giving details of our investigation will be available on our website in the coming days.”
