A London Midland class 321 pulls into Bricket Wood station with a service to Watford Junction. Credit: Peter Alvey Photography (Image: Archant)
Trains running on the Abbey Flyer service may be cancelled or delayed by up to 30 minutes, London Midland have warned.
This is due to speed restrictions on Watford Junction, meaning trains are having to run at 20mph.
It is believed it has something to do with the 30C temperatures which have been beating down on Hertfordshire.
If services are cancelled, London Midland say replacement transport will be made available.
Arriva buses will also accept London Midland passengers on the 321 service.
The disruption is expected until 6pm this evening.
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