A significant landslip near the Harbury Tunnel has led to the closure of railway lines between Leamington Spa and Banbury.
The lines will remain closed for at least one week to allow for an investigation to determine the scale of the problem.
After an initial assessment, it is estimated that around 350,000t of debris will need to be removed from the site.
Engineers have been monitoring the site to see if it continues to move and to determine if it is safe to begin a detailed assessment, which may take several days.
The site is a known problem area, experiencing elevated levels of ground water.
A similar landslip incident occurred near the Harbury Tunnel in February 2014, resulting in rail service cancellations and delays.