Train passengers were stranded on trains and at stations for several hours after overhead electric wires were damaged.
Passengers had to be evacuated by police from a number of trains as the disruption unfolded last night.
The problem began shortly before 7pm on Tuesday near Cambridge, leading to several lines serving the city being blocked.
Operator Great Northern released a photograph showing a pantograph - a device mounted on train roofs to collect electricity from wires - on the ground next to a train.
Another image shows loose electric wires draped over the side of a train as the firm confirmed the power was switched off.
British Transport Police said it assisted Network Rail in 'evacuating passengers from several stranded trains', working with operators Great Northern, CrossCountry and Greater Anglia.
Passengers were stranded on trains and at stations for several hours after overhead electric wires were damaged. Pictured: A damaged pantograph lies on the ground next to the train
British Transport Police say they were on hand to evacuate passengers from trains last night
Passenger Colin Chang sent a tweet to Greater Anglia at 10.54pm which read: 'Was meant to get back to Cambridge North at 18:58 but now having to get an Uber to get there.
'Being stuck on the train for 3+ hours with bad communication is not a great look. I've already filed for compensation but I'm not optimistic. Ruined my day.'
Another frustrated traveller, named Lee, wrote a message to CrossCountry stating: 'Can you please explain why, given that I've been sat on your train from Stansted to Birmingham for over three hours when it should've arrived at my destination almost two hours ago, I haven't been offered the alternative of a taxi to my destination?'
The firm replied that taxis were waiting at Cambridge for passengers on his train to take them to their destinations.
Ben Connor sent a tweet to Great Northern which read: 'Is there any information on when more than one or two taxis, or a bus will be arriving at Royston.
'Have been here for 1.5 hours trying to get to Cambridge, there are dozens of people here stranded.'
The service continued to be affected through Wednesday morning
Pictures shared by Great Northern shows loose wiring draped over the top of a train last night
An update issued by Great Northern said: 'Engineers have completed repairs on the overhead wires and are now carrying out checks and testing before the lines are reopened.
'This is expected to take several hours, so trains remain unable to run to/from Cambridge.'
The operator has since confirmed the tests are complete and said the line has reopened but that disruption is ongoing.
Passengers are advised to check before they travel with delays not expected to end before 1pm.