On August 8th, 1963 a gang of 16 bandits robbed a train in Britain carrying Royal Mail from Glasgow to London. Surprisingly, this didn't play out like a Clint Eastwood western movie. The group didn't have a single gun (or horse). Using a little ingenuity, numbers, and some inside information they were able to stop the train and make off with over £40 million (adjusted for today's value). The only reason that much money was being transported was because of a banking holiday, which meant two days worth of cash was being moved. Some of it was even scheduled to be destroyed, because it was old.
The gang was able to stop a massive locomotive by attaching a 9 volt battery to one of the red stoplights along the train's route. When an engineer stepped out to check the tracks he was overpowered by the gang, who then rushed the remaining engineer still in the engine car. The gang then easily entered the Royal Mail cars, as there was no security, and drove the train to a nearby rail bridge overlooking a road. They unloaded an estimated 2.5 tons of of mail and cash into an old army truck and drove to a nearby farm they had recently purchased as a hideout. They split the money evenly and left the hideout the next day, fearing the authorities were on their trail. The cops did eventually find the hideout, and some evidence, but not enough to reel in the entire gang.
Furthermore, only a very minuscule amount of serial numbers on the stolen cash were recorded, so it was almost impossible to follow a trail of stolen cash. However, the first two criminals to be arrested were tipped off to police by their landlord. They had paid for three months rent in cash, using only 10 shilling notes. The landlord became suspicious, likely hearing about the crime and denominations of the bills on the news, and then contacted the police.
The authorities weren't able to capture the entire gang of bandits, and to make matters worse, some of the criminals escaped from prison shortly after being caught. Lastly, most of the money was immediately laundered by the criminals, so very little of it was ever recovered. I'd like to think that this incident probably led to some serious reform within the British police and penal system. (Credits to BBC and a link to the criminals below and photo credits: http://www.express.co.uk/news/uk/419504/The-men-who-hunted-the-Great-Train-Robbers)