A Hartlepool woman has described the terrifying moments as her train derailed and was hit by another.
Sarah Lowther was sitting near the front of the 6.19am London Midland service from Milton Keynes to London Euston when it came off the track at around 7am after hitting a landslide.
Radio presenter Sarah was on her way to work when the accident happened: "At 5.16 this morning, we had an unbelievable thunder and lightning storm," she said.
"When I was driving from home to Berkhamsted station, I couldn't see the road, so I was surprised the trains were running.
"About ten minutes into the journey, the train hit something. I thought 'Crikey, I am air-borne. It's the first time I have flown in a train.'
"I tucked myself into the brace position and hugged the seat. Within seconds, the on-coming train hit us. Our train had derailed into the path of the oncoming train.
"We were doing 70 miles an hour and the other train was doing 75 - it would have been faster if we weren't going through a tunnel at the time.
"Our driver Mark saw the landslide 30 ft before he got to it. He managed to put the brakes on and we braked inside the tunnel."
Sarah, 49, praised the behaviour of passengers throughout the crisis: "There was a Dunkirk spirit. When the first collision took us out of our seats, people just said 'Oh, - there was no screaming.
"Then all we heard was a voice asking if everybody was alright.
"We tried to get to the driver but the door to his cab was locked. We could hear him calling the signalman."
Passengers had to wait for several hours to get off the derailed train: "Because we were in the tunnel, people did not know we were there for a while.
"We could not walk along the tunnel, because it was not safe so we had to wait for a train to come and get us."
Staff used a wheelchair ramp to bridge the gap between the two trains.
Sarah was taken ill after getting onto the rescue train, but has been given a clean bill of health.
"We got across and it was then I thought I was not feeling too well. There were paramedics on the rescue train - they gave me an ECG and I am alright," she said.
"The man next to me had a bad back injury and the driver had a poorly,back too.
"A few of us have probably got whiplash and we were very dehydrated, but generally, we are remarkably unscathed, given we were doing 70 and the other train 75.2
Sarah's mum and dad Ray and Barbara still live at Hart Station and were the first people, she rang.
"The first call I made was to Hartlepool," she said.
"The first thing I did was to call my mum and dad - I'm glad I didn't get through, because I was in a bit of a state.
"Why would I ring them at seven to tell them I was involved in an accident they couldn't do anything about?"
Call for investigation
Now Sarah is calling for an investigation into the maintenance of the rail line: "Through the last two years, there has been deforestation on the side of the railway - they have been cutting down the trees so there are no leaves on the tracks.
"But no trees means no roots to hold the soil together."
The incident caused major delays to services and London Midland warned passengers to avoid travelling to or from London Euston, the sixth busiest station in Britain.
Martin Frobisher, route managing director for Network Rail, said a train hit a landslip, resulting in a small section of the train leaving the tracks a few miles north of Watford. The train remained upright, he added.
"Engineers are on site and train services are now running through the area but it will be some time before a normal timetable resumes," he said.
Initially, all four lines of the West Cost main line were closed at the scene, but two were reopened at around 8am.
Mr Frobisher added: "Our priority is to fully reopen the railway as soon as it is safe to do so. A full investigation into what happened will take place."
A London Midland spokesman said: "We can expect disruption until at least the afternoon so the advice is not to travel to or from Euston this morning and check for further updates later today. London Midland train tickets are being accepted on alternative routes."
British Transport Police said officers were working with fire and ambulance services at the scene.
Passengers were advised to check www.nationalrail.co.uk for the latest updates on services.
Francis Thomas from London Midland told BBC Radio 4's Today programme the driver of the train "reported hitting something" which caused the train to derail.
"This is the busiest piece of railway that carries freight and passengers in the whole of Europe, it affects anything that goes to the West Midlands, to the North West and Scotland - all train services have been affected this morning, it will take quite a long time to get the service recovered," he added.
"We have got extra staff on site assisting those passengers (on the train) and that is our focus - to look after those passengers, make sure they are all right and get them to their final destinations."
Mr Thomas said trains go in and out of London Euston every 90 seconds, and that he expects disruption "until lunchtime, and possibly longer".