A train has hit an obstruction between St Albans City Station and Luton, creating the risk of delays and cancellations.
The 1318 Luton to Rainham service hit an object on the track and is currently at a stand at St Albans, blocking a southbound line.
It is not yet known what the object is, but it has been reported as being metallic.
Trains may be cancelled or delayed by up to 30 minutes and there may be platform alterations at St Albans and Radlett.
The disruption is expected to last until 3.30pm.
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