Trains are now running in and out of London again following earlier cancellations - but with severe delays.
Trains on the Birmingham line - which stop at Seer Green, Gerrards Cross, High Wycombe and Saunderton - were being cancelled but are now severely delayed, causing rush hour chaos for commuters.
Emergency services are dealing with an incident at Sudbury and Harrow Road.
Delays and cancellations are expected for up to 90 minutes, while tickets are being accepted on the London Underground.
All trains out of Marylebone heading north will call additionally at West Ruislip between 6pm and 7pm.
Chiltern Railways is advising passengers travelling in and out of London Marylebone to use the Metropolitan line through Rickmansworth and Amersham.
More information as we get it.
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