Travellers are facing chaos on the east coast line after a sinkhole opened up on the railway line near Leuchars.
Services between Edinburgh and Aberdeen have been disrupted and Scotrail has arranged replacement bus services.
Network Rail said that ballast stones had subsided as a result and engineers now need to assess the depth of the subsidence.
A spokesman said: "A sinkhole has caused a dip to open up below one of the lines at Leuchars.
"It's around a metre deep and will require three tonnes of ballast to fill."
Services between Edinburgh and Perth, Kirkcaldy and Glenrothes will run as normal.
Trains between Aberdeen and Edinburgh will only run between Aberdeen and Dundee in both directions.
Travellers are urged to check on their rail operator's website and social media channels for updates.
Disruption is likely to last until the end of today.