It appears to only be affecting
GWR▸ , so I assume it's not related to the earlier land slip near Sand Tunnel, or the recent works at Guildford. Fortunately the work is being done during the next few nights, so most services are not affected.
Urgent repairs to the railway are required between Guildford and Redhill. As a result, until Saturday 4 January, the line between these stations will be closed each evening from 23:20 until 06:20. On Saturday evening, the line will close from 23:00 until 10:00 on Sunday 5 January.
During the times in which the line closures are in place, Great Western Railway trains will be unable to run between Guildford and Redhill.
Replacement buses will run in place of cancelled services between Guildford and Gatwick Airport.
Customers are advised that coaches will run later that than the advertised train times, due to line of route and additional time required for loading and unloading.
https://www.nationalrail.co.uk/service_disruptions/239294.aspx
