On May 22, 1915 an error with the Quintinshill signal box (pink) near Gretna Green in Dumfriesshire, Scotland caused a southbound troop train (green) to smash into a stationary train (orange) and then a northbound sleeper train (purple) to hit the southbound one. Trains holding goods (grey and blue) were also nearby. Gas from the lighting system of the troop train ignited and set all the trains on fire. Around two hundred died, mostly men in the troop train. It was the worst rail disaster in British history at the time.
Image: Gif from By Emoscopes, Tevildo [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
So what's the conspiracy angle? The signalmen present that day were tried and convicted for culpable homicide, released from jail the following year, and re-employed by the same railway company (though not as signalmen). This led to speculation that the railway company and the government (who was running the railways in war time) used the signalmen as scapegoats, letting them take the fall for what was actually the rail company and government's responsibility. (Wikipedia)
Another controversy was that some of the troops trapped in the blaze were either shot (mercy killings) and/or committed suicide. A BBC documentary was released that confirmed the mercy killings in 2015.
Quite a sad story, no matter who was responsible.
Q is one of the hardest letters during the A to Z challenge. What's your Q today?