Problems on rail line between Watford and London Euston
Train services between London Euston and Watford Junction are currently affected by a fault on the signalling system.
As a result, fewer services are running, while trains running to and from these stations may be delayed, altered or diverted.
The issue is believed to affect services which call at stations including Apsley, Kings Langley, Hemel Hempstead, Berkhamsted, Tring, Cheddington, Leighton Buzzard, Bletchley and Milton Keynes.