Police rehearsed a knife attack scenario on a train line in March - here's what went differently this time

British Transport Police held an emergency exercise for press officers in March, which ironically involved a stabbing on a train travelling south near Huntingdon.

In the training drill, the train stopped immediately between stations when a passenger pulled the emergency cord. It took police 25 minutes to reach the train and casualties, far longer than the eight minutes in which Cambridgeshire firearms officers reached the scene at Huntingdon station.

Follow latest: 'Nothing to suggest' terror attack, police say Chris Webb, a crisis communications expert who helped run the exercise, said: "People think if you pull the emergency cord on a train it stops immediately, but that's not what happens these days. "As soon as the driver knows there is a problem, he or she radios the line operator HQ and they discuss where to stop.

"The decision last night was to keep going to Huntingdon station, where it was much easier for armed police to get on." He added: "It must have been awful for passengers when the train kept going for another ten minutes or so. "It's always a balance.

It might have prolonged the attack, but stopping in the middle of nowhere can mean the attack stops but it's much more difficult for the emergency services to get there." Mr Webb, former head of news at Scotland Yard, said such exercises are held regularly by train operators. A similar drill was carried out on the London Underground weeks before the 7/7 bombings in 2005.

"There are always lessons to learn but you cannot guard against everything." In the training exercise in March the suspect was a white man with mental health issues. He was shot dead by police.

Read more:What we know so far about the Huntingdon train stabbingsEyewitness recalls horror attack on high-speed train What happened in the Huntingdon attack? Police triggered the Plato code to all emergency services in their initial response to the Huntingdon train stabbing, but that did not label it a terrorist attack. Plato is called for a major incident where it's thought a suspect is on the loose and has already, or is liable to, cause serious injury.

Plato does not denote a terror attack, though it is often used in terrorist incidents. In a Plato response paramedics, fire fighters and other first responders are sent to a safe rendezvous point while armed police go in and deal with the suspect.

Plato depicts a situation where unarmed responders are vulnerable and are kept back until it is safe to approach casualties. There are exceptions and it's understood the East of England Ambulance Service has a special Hazardous Area Response Team (HART) which was allowed to accompany armed police onto the platform where the two suspects were arrested last night.

Once other first responders were allowed in, Plato was called off - an important part of the operation. Plato was called during the initial response to the Manchester Arena bomb attack in 2017, but the fire service was not told it had been called off for two hours and that meant its officers did not go in to help with the rescue..