@Tom
Disruption across the entire network is more than possible if a train is on fire.
The trains at points beyond Heworth (Pelaw - Sunderland or Pelaw - Shields), possibly can't get back into Heworth, Gateshead etc).
Electricity may be switched off, depending on the nature of the fire - it could have caused issues with signalling etc.
As well as that, you have trains on the other side of the river heading towards Heworth, need to find somewhere to terminate and get turned around, so they can continue their journeys back to Airport or St James.