Major disruption is impacting trains at Stevenage and Welwyn Garden City stations this afternoon.
The problems started when a train developed a fault at Stilton Fen, between Peterborough and Huntingdon, and blocked the 10.53am service from Peterborough to London Kings Cross.
While the issue was investigated, overhead lines were switched off and the trapped carriages were sent back to Peterborough.
All lines have now reopened there and the faulty unit attached to a rescue train.
However, residual disruption is being prolonged after trespassers were found on the railway at Welwyn Garden City - due to this, services may be diverted via Hertford North.
National Rail predict disruption for passengers until at least about 9pm.
People should expect two hour delays, cancellations, or short notice alteration.
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