The driver of a high-speed train where a mass stabbing took place said he had "probably about 10 seconds" to decide to switch to the slow line before making an unscheduled stop.
Andrew Johnson was driving the London-bound LNER service on 1 November last year when the train stopped at Huntingdon in Cambridgeshire amid a mass attack by Anthony Williams.
Giving evidence, Johnson told jurors at Cambridge Crown Court he heard someone on the intercom "screaming 'there's a knifeman stabbing people'".
Williams, 33, of Peterborough, denies 10 counts of attempted murder on the train.
He is also charged with four other attempted murders following earlier incidents in Peterborough and London.
Johnson told jurors that ordinarily the next stop after Peterborough would be Stevenage, a journey that takes "exactly 30 minutes".
He said when they were around Abbots Ripton in Cambridgeshire and "as it was uphill" the train was travelling at 122mph, adding the service was "bang on time".
Johnson told jurors there was an unusual alarm "which indicated someone was trying to open one of the passenger doors".
"I have five seconds to override or the train will stop immediately," he said.
He said if the train stopped it would take about a mile or 30 seconds to stop.
In Court: Huntingdon Train Attack
Follow all the developments from the trial as they unfold on our BBC podcast In Court
Johnson said he picked up the intercom to that door but received no response and thought "Saturday night, someone's messing around".
He said while he was speaking to the train manager to ask her to investigate "someone over-spoke us" and he heard someone "screaming 'there's a knifeman stabbing people'".
He thought the "best thing to do was to get to a platform" and the voice of the individual "really drove that home".
Johnson said he had "probably about 10 seconds" to switch to the slow line, adding: "I just knew that was the right option, it just came instinctively."
He pressed the urgent button and told a signaller "get me to Huntingdon and get me the police", stating if he had pressed the emergency one it would have stopped all trains in the area.
The train driver said he adjusted his speed while the signaller had to wait for two minutes in order to switch the points.
Jurors heard Johnson "could hear banging and screaming" on the cab door, and he said: "It was incredibly loud, and people were begging for me to open the door as well."
Johnson told jurors that if he left his seat "after a short period of time the train will stop".
"My mind was completely focused on one thing and that was to get the train to Huntingdon," he said.
When the train pulled into Huntingdon at 19:44 GMT, the court heard he had to override the system to open the doors because it was "not a programmed stop for us", taking about 15 seconds.
He said he saw passengers on the platform and "they didn't just run, they sprinted like I'd never seen before".
The court heard that throughout Johnson never saw Williams.
As well as arguing Williams was attempting to murder the 10 people on the train, prosecutors allege that on 31 October he tried to kill three people in Peterborough.
He is also charged with attempting to murder a teenager in the early hours of 1 November on a DLR train in east London.
Whether Williams carried out the attacks is not in question – the court has been told – but jurors must consider whether he intended to kill.
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