William Dempsey, 29, left two Army camouflage bags containing an Uzi sub-machine gun with silencer, a handgun and a sawn-off double-barrelled shotgun at Carlisle station as he stepped off the Glasgow-bound train.
The then mortarman with the 5th Battalion, Royal Regiment of Scotland, was dressed in desert combat uniform when, exhibiting "somewhat bizarre behaviour", he told rail staff he was being followed by police officers and there were snipers at the station.
Dempsey, from Paisley, had collected the firearms from his friend, James Ashdown, 32, in Canterbury, Kent, where the soldier's barracks were, and they were destined for underworld figure Barry Kelly, 34, and his "lieutenant", Craig Colquhoun, 28.
Today, Dempsey, of Woodhead Gardens, and Ashdown, of no fixed address, were each jailed for eight years at Liverpool Crown Court after they both pleaded guilty at earlier hearings to conspiracy to buy or sell illegal firearms. Drug addict Ashdown received an additional three years for possession and intent to supply cocaine in a separate case.
Kelly, of Dalquhorn Avenue, Darvel, and Colquhoun, of Murray Place, Barrhead, were jailed for nine years and seven years respectively after they were found guilty by a jury earlier this month of the same firearms offences.
On August 17 last year Dempsey also left his identification papers and 194 rounds of ammunition in the bags, which had his name embroidered on them, the court was told.
He had a final pint in a local pub before he was inevitably arrested as the subsequent investigation uncovered his fellow conspirators.
Dempsey confirmed to British Transport Police (BTP) that no weapons were missing from the armoury at his barracks and officers began to suspect that, rather than acting alone, he was part of a supply chain moving firearms around the country.
Dempsey's mobile phone was subjected to in-depth forensic analysis which revealed messages between him and Ashdown regarding large cash payments for "toys" - items believed to be the firearms seized at Carlisle, said BTP.
Further investigation revealed links to both Colquhoun and Kelly. On August 16 last year Colquhoun was captured on CCTV at the Royal Bank of Scotland in Paisley paying £3,500 into Dempsey's account.
Officers subsequently arrested Kelly, Ashdown and Colquhoun and during the arrests found £20,000 hidden in the loft of Kelly's house. Mobile phones were also seized and analysis of these cemented the links between the four conspirators.
Prosecutor Nicholas Kennedy said the provenance of the Uzi and the handgun was unknown but the shotgun had been stolen in a burglary at farm buildings in Maidstone in February 2007.
Sentencing the four, Judge Robert Warnock said: "I am wholly satisfied that they (the weapons) were destined for use in serious crime and had a clear potential for causing serious injury and death.
"Had these firearms reached their intended recipients, I have no doubt they would have significantly contributed to the upward spiral of gun crime in this country."
Addressing Dempsey, the judge told him he had used his uniform to hide what he was doing on the day he was arrested.
"For reasons principally to do with your abuse of alcohol and drugs, you abandoned the firearms at Carlisle station," he said.
"I make it clear, however, that these actions were not motivated by remorse or insight but your befuddled and confused thinking brought on by your alcohol and drug abuse and your consequential paranoia and instability."
The judge told Kelly he was "clearly a relatively sophisticated criminal who is well-versed in the use and abuse of mobile phone technology".
His "employee" Colquhoun was a peripheral player in the conspiracy, he said.
Ashdown was an active street dealer of drugs who was motivated to make money to fund his cocaine habit, the court heard.
Mitigating for Dempsey, David Thompson said his client had joined the Army at the age of 17 and had served in Northern Ireland, Iraq and Afghanistan.
He was described as a "reliable and competent soldier" but his life began to go downhill last July, said the barrister.
Dempsey had had long-term problems with alcohol abuse and had subsequently been diagnosed with post-traumatic stress disorder after his battleground stints.
He had begun to dabble in cocaine and had accumulated a drug debt of £5,000 at the time he entered the firearms conspiracy, the court heard.
Mr Thompson said: "The pattern of Mr Dempsey's life over the past 12 months leading up to this incident is one of general decline - decline in his psychological make-up, his health, his use of drugs and alcohol... and he was at a point when he was vulnerable, prone to poor use of judgment and bad decision-making."
His problems worsened and he was exposed to influences in his home town of Paisley - which he had left and joined the Army to get away from such problems when he was younger.
Dempsey became paranoid as the train journey progressed towards Glasgow, said Mr Thompson, which he said could partly be attributed to his stress disorder.
"He reached the point in Carlisle where he simply could not carry on," he continued, "and in effect abandoned the consignment at the rail station and disappeared into town.
"He tells me he knew he was going to be arrested and was enjoying a final pint of beer."
Mr Thompson said it was "a crying shame" that the defendant had become involved in a conspiracy "diametrically opposed" to the 12 years of service he had given to his country.
"He does have real regrets for what he did," said the barrister. "He is filled with remorse for getting involved and the shame he has brought upon his regiment."
The court heard that Kelly maintained his innocence.
He had eight convictions including fines for public order offences and illicit drugs possession but had never served a jail sentence.
Colquhoun had previous convictions between 2002 and 2012, including a 20-month jail term for assault imposed at Paisley Sheriff Court last November - a sentence which he is still serving.
Ashdown was a cocaine addict who wanted to sell the firearms and ammunition and used Dempsey and his connections in Paisley to do so, the court was told.
He had not personally met the purchaser, Kelly, or Colquhoun during the conspiracy.
As he was led to the cells, Dempsey claimed his co-defendants, Kelly and Colquhoun, had been victims of police corruption.
He shouted: "Barry Kelly and Craig Colquhoun have been set up."
Following sentencing, Detective Chief Inspector Simon Taylor, who led British Transport Police's investigation, said: "These four clearly intended to put illegal firearms on to the streets of Scotland and it is a fair assumption that they would have been used to commit criminal acts or to intimidate people.
"In uncovering and unravelling the complex conspiracy BTP, in partnership with Kent Police and Police Scotland, have taken this potential threat out of circulation and ensured the conviction of four dangerous men.
"It is rarely that we see any incidents involving firearms on the railway, but this case clearly shows we are equipped to deal with the threat and any investigation which follows and sends a clear message to the criminal fraternity that weapons have no place on the railway - or in wider society.
"Police forces will work together to seize weapons and put all offenders before the courts."
Mr Taylor added: "This year-long investigation has provided a stern test for British Transport Police and the Crown Prosecution Service and I would like to thank all who have dedicated countless hours to the initial investigation and in building the strong case which has seen Dempsey, Ashdown, Kelly and Colquhoun convicted for extremely serious offences."
Fran Gough, Senior Crown Prosecutor for the North West Complex Casework Unit, said: "We have worked closely with the British Transport Police to build a strong case against the defendants to successfully break down this firearm and ammunition supply chain. We would also like to thank the Scottish police for their assistance in the case.
"Evidence proved that James Ashdown was the source of the weaponry, William Dempsey was the courier, Barry Kelly was the purchaser, whilst Craig Colquhoun was Kelly's right-hand man.
"These dangerous individuals and their weapons were intercepted and have now both been removed from the streets where they can pose no further risk.
"The CPS would like to reassure the public that we will robustly prosecute those who seek to supply deadly weapons into the communities of the UK."