Virgin Trains East Coast said the 11.23 train to King’s Cross from Leeds had been cancelled.
Passengers can travel on the 11.45 to King’s Cross, changing at Doncaster for Grantham and Stevenage.
The train operator said all lines were blocked because of the incident, which took place between Welwyn Garden City and Potters Bar, with disruption expected until 12 noon.
Hull Trains said the 8.23 Hull to King’s Cross Service had been delayed 58 minutes.
Trains may be cancelled or delayed by up to 90 minutes.
Other services affected include Grand Central and Great Northern.