Trains between Hitchin and Cambridge are experiencing delays of up to 20 minutes after a cow wandered onto the tracks near Meldreth.
The cow was ushered off the line and staff at National Rail are currently searching for the farmer.
National Rail is unaware how the wandering animal got there.
There are further delays on the same line after a HGV lorry struck a bridge in Cambridge Road, Hitchin.
The lorry is being removed and the bridge is being examined.
Story update: 16:30 - trains are now running on time
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