The line between St Albans Abbey and Watford Junction has been closed due to a broken-down train.
Trains running to and from these stations have been cancelled as a result. Tickets are being accepted on the Arriva 321 service between the two stations.
The disruption is expected to last until 5.30pm.
Train operator London Midland have reminded travellers they can claim compensation if their train is delayed by 30 minutes or more.
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