Two LNER trains became stranded - one of them in a tunnel - after overhead power cables failed between Grantham and Newark Northgate stations.
The power failure caused disruption throughout the route and many services had to be diverted to other stations including Peterborough and Lincoln.
Some passengers on the two stranded services have been posting messages on social media saying conditions were "extremely hot" onboard due to the fact there was no air conditioning.
One post stated that two elderly women passengers had fainted due to the uncomfortably hot conditions onboard.
British Transport Police Cambridgeshire tweeted to say that they would be assisting with a train evacuation, just north of Grantham.
An LNER spokesperson told ITV News Central that locomotives that don't rely on overhead power cables had been sent to move the two trains.
The spokesperson said Network Rail engineers were working to restore power to the affected section of the line.