The first was a string of 38 empty coal wagons coming south, signalled away from Welwyn at 23:20 behind a tender locomotive. Somewhere inside Welwyn North Tunnel the engine failed. Guard Wray suggested rolling back down the falling gradient to Welwyn; the driver refused - it would have been dangerous and against the rules. Wray was then meant to walk back along the line and place detonators on the rail to warn following trains. He did not. He also did not telegraph either signal box to say the line ahead was blocked. The second train, a Midland Railway goods working from London hauling 26 wagons, stopped at Welwyn at 23:36. The Welwyn signalman, who had never received the 'out of section' clear from the coal train, telegraphed Knebworth to ask whether it had cleared the tunnel. The reply was the misread needle-code. He believed he had been told yes. He pulled the signal. The Midland goods went into the tunnel at speed. The third train, a Great Northern express freight carrying meat from Scotland for Smithfield Market in London, was let in behind it before anyone above ground understood what had just happened.

When the Midland goods rounded the curve inside the tunnel and saw the stationary coal wagons, there was no time to brake. The collision killed Guard Wray instantly. It severely injured a man named Rawlins, an employee of the Metropolitan Railway who had been riding in the brake van in breach of Great Northern rules; he died on the morning of 12 June. The driver and fireman survived, trapped in the wreckage. Before any of them could climb out and reach a signal box, the meat train arrived, struck the burning wreckage of the first collision, and the whole tangle caught fire. Because the wreck sat directly beneath one of the tunnel's tall ventilation shafts, the shaft drew the flames upward like a chimney. The fire burned for two days. Rolt, writing nearly a century later, quoted the locals: 'all that night and all through the next day the ventilation shaft belched flames, smoke and the smell of roasting meat over the surrounding countryside.'

Wray and Rawlins were the only deaths - a smaller toll than the damage might suggest, though the destruction of locomotives and rolling stock was enormous. Captain F H Rich of the Royal Engineers conducted the inquiry. His report, in the careful prose of Victorian disaster investigations, placed the principal blame on Guard Wray for not protecting the rear of his train. The misread telegraph code was named as a secondary cause. But Rich, like the best Victorian engineers, was not interested in punishing the dead. He was interested in what the system itself had failed to prevent. His report became one of the foundational documents of British railway signalling reform.

Rich recommended two specific changes, and the industry adopted both. First, before any train could enter a section of line, the signalman in front had to be positively asked for, and positively grant, permission - rather than the loose 'out of section' system in which one acknowledgement covered both the previous train clearing the line and the next one being allowed in. Second, a dedicated block telegraph instrument, distinct from the general-purpose 'speaking' telegraph, should permanently display the state of every section: occupied, clear, line blocked. This is the absolute block system, and in various refined forms it became the foundation of safe single-line and multi-track working across Britain. Every rail crash that did not happen across the next century - because a signalman could glance at a needle dial rather than guessing - traces back, in part, to the smell of roasting meat that drifted across Hertfordshire in June 1866.

Above the tunnel today, the world is gentler. Welwyn North Tunnel is still in use - East Coast Main Line trains thunder through it dozens of times a day on the run between London King's Cross and Edinburgh. The village of Welwyn (the station now called Welwyn North) sits in its valley, almost wholly unrelated to the larger Welwyn Garden City a few miles south. The ventilation shafts that once drew the fire skyward still stand among the trees as low brick towers, easy to miss if you don't know what they are. Two men died in the smoke below them, but the principle their deaths produced - one section, one train, ask first - has kept countless others alive on the railways of Britain ever since.