St Albans firefighters rushed to a blaze involving 400 tonnes of straw this morning.
Six firefighting crews - from Potters Bar, Cheshunt, St Albans, Borehamwood and Hatfield - were called to the scene of the incident at a farm on Cattlegate Road in Northaw.
The blaze started at about 5.15am, and spread to trees, hedgerows, and an adjacent manure heap.
Farm vehicles had to be removed from the area and resultant smoke is affecting trains through Cuffley Station.
National Rail has warned passengers on the Letchworth Garden City train line via Hertford North should expect "severe delays".
Herts Fire and Rescue Service station commander for Dacorum and St Albans, Lee Slipanczewski, Tweeted that the crews are currently dampening down the area.
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