A derailed freight train between Sunderland and Hartlepool is causing serious disruption between Newcastle and Middlesbrough.
Network Rail says it is currently mobilising specialist teams to attend and inspect the freight train.
Trains from Sunderland cannot run towards Hartlepool, but will instead divert via Newcastle and Darlington.
Trains from Hartlepool towards Sunderland may be delayed by up to 15 minutes.
Network Rail anticipates disruption will continue until at least 3pm Wednesday.
Train operator Northern says customers will be able to use their tickets with other train operators to travel in both directions.