A COMMUTER train travelling at almost 100mph left passengers shocked after its coupling failed and the train split in two. Train operator First Capital Connect has launched an investigation to discover what caused the 5.15pm from London King's Cross to Peterborough train to fail. The incident happened on Thursday, about 10 minutes after the train had left London bringing commuters back to Huntingdon and St Neots. The eight-carriage train, with approximately 700 passengers on board, was on the fast track just south of Potters Bar when the incident happened. The two, four-carriage units, which made up the train, became detached. The emergency brakes were automatically activated and the two halves of the train screeched to a halt less than a carriage length apart. Commuters were stranded for an hour as engineers attended the scene. Passengers in the front half of the train reported being kept informed of developments by the driver but those in the rear half had to use mobile phones to find out what was happening. Commuter Ann Doyle, 50, from Cambourne, was travelling in the rear half of the train. She said: "We didn't know what was going on. We thought the front half of the train had gone off up the track and left us behind. "People were worried we might be struck from behind by another train and when we tried to phone the train company we got mixed messages about what was going on. "The first we knew was when a man in a fluorescent jacket got on after about an hour and came on the tannoy telling us we'd be on our way soon." The train eventually reached St Neots station at approximately 7pm - more than 75 minutes late. A spokesman for train operator First Capital Connect said: "A thorough investigation is underway to investigate the cause of this worrying and extremely unusual incident. "Our 1707 service from King's Cross to Peterborough was on the fast lane of the line when the coupling between the two, four-carriage units became detached. "We offer our sincerest apologies for the delay and anxiety caused to passengers, although we can confirm that at no point during the incident were they in any danger. "When the carriages became uncoupled our network computer showed there were two separate 'trains' in the same area and the emergency brakes were automatically applied to both halves of the train." Mrs Doyle added: "People did see the funny side but the uncertainty was worrying. I'm just glad we weren't in a tunnel as I hate confined spaces. "I won't have any problems getting back on the train though. Those of us who work in London know that whatever happens you just have to get on with it." The investigation into the incident is expected to be concluded this week. INFORMATION: Customers affected by the incident can contact First Capital Connect customer services on 0845 0264700.