Trains on the route from Lancaster to Skipton are severely disrupted after a mudslide damaged track this morning.
There are no trains in both directions between the two stations, although they are still running on the section between Skipton and Leeds via Bingley.
The landslip occurred near Bentham, and trains cannot serve stations between Lancaster and Skipton, including Bentham, Clapham, Giggleswick, Long Preston, Hellifield and Gargrave.
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Network Rail and train operators Northern are unable to confirm when repairs will be complete.
A spokesman for Northern said: "The mud has completely covered the track in both directions meaning we cannot run trains between Lancaster and Skipton.
"The mudslide was discovered at around 7.45am when a Lancaster to Leeds service encountered problems on the line.
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"We are working with Network Rail to get the track cleared as soon as possible but, given the amount of mud and debris on the line, this could take some time. We apologise to any customers whose journeys are disrupted and ask anyone planning to travel on this route to seek alternative arrangements."
Northern is in the process of trying to arrange rail replacement road transport to help keep customers on the move.
Back in July, heavy rain and flash flooding in the Dales caused a landslip between Kirkby Stephen and Garsdale which halted trains on the Skipton to Carlisle line.
Between 2016 and 2017, the Settle to Carlisle line was badly affected by landslip and flood damage, leading to part of the route being closed for several months while repairs were undertaken,