Cork was, in the words of one study of the period, by far the most violent county in Ireland during the War of Independence. Several active guerrilla brigades operated across its hills, and the West Cork 3rd Brigade was among the most effective. British forces had taken to moving troops by train because the roads of the county were too dangerous and too easily blocked. The IRA took notice. A week before Upton, a different IRA party had successfully attacked a troop train near Drishanbeg, killing a sergeant and wounding five soldiers. The pattern looked workable. Trains held intelligence, weapons, and prisoners. They also held something else - regular civilians, going about their business, who could not be cleared from the carriages before the shooting began.

When the train arrived at Upton on the morning of 15 February, the IRA's intelligence had already collapsed. About fifty additional British soldiers had boarded the train at Kinsale, mingling with civilian passengers throughout the carriages. The two missing scouts could not warn anyone. One IRA man saw the soldiers board and pedalled hard to Upton, but he arrived two minutes after the shooting had started. The fight lasted only ten minutes. By the time it ended, three IRA volunteers were dead and two more wounded. Six British soldiers were injured, three of them seriously. Charlie Hurley, leading the ambush, was struck in the face by a bullet but survived. And at least eight civilian passengers were dead and ten wounded - including two commercial travellers killed by the very first volley. The New York Times wrote of the train that a shower of bullets was rained on the train, practically every compartment being swept.

Upton fell inside what Tom Barry would later call the twelve dark days of the 3rd Cork Brigade. Between 4 February and 16 February 1921, eleven brigade members died. Charlie Hurley himself had nearly been killed on the 4th when British troops raided a safe house. Another man died in an accidental shooting on the 7th. The Coffey brothers were killed in their beds on the 14th by Black and Tans or Auxiliaries. Three more died at Upton on the 15th. On the 16th, the Essex Regiment found four men digging a trench at Kilbrittain and shot them all. Of the eleven dead, only the three at Upton died in combat. Hurley would carry the failure of the ambush for the rest of his short life. Barry remembered that Hurley mourned deeply for his dead comrades of the Upton train attack and for the dead civilians. Hurley died at Crossbarry a month later. The Upton ballad, The Lonely Woods of Upton, written in the 1960s, would carry his memory and the names of the dead.

Only a month after Upton, at the Headford Ambush in neighbouring Kerry, another IRA column attacked another troop train. Again there were civilian casualties beside the soldiers and the volunteers. Upton became a cautionary case for IRA planners. Attacking soldiers who could not be separated from civilian passengers meant accepting that ordinary people - farmers heading to market, shopkeepers riding home, a woman travelling with her child - would be the ones absorbing some of the bullets. Some of the volunteers who knew the dead never quite recovered from it. One of the three IRA men captured at Upton is reported to have provided information that almost led to the encirclement and destruction of the entire 3rd Cork Brigade column at Crossbarry the following month - the engagement at which Charlie Hurley himself was killed. The eight civilians of Upton became the bedrock human cost of choosing this kind of war.