LNER reported the incident at 9.40am, which happened between Peterborough and Stevenage.
All services from Leeds to London are likely to be cancelled or delayed as emergency services attend the scene.
The 10.28 departure from Wakefield to London Kings Cross has been cancelled, as well as the 10.33 and 10.57am departures from London Kings Cross to Wakefield.
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Delays are expected until at least 2pm.
The Samaritans can be contacted on 116 123 from any phone for a confidential chat.