First, perhaps a technicality, but the train wasn't moving at all by the time the gang launched their robbery. As noted by the gem author below, the rail was loosened and pulled askew, derailing the train. This was the perfect location to set in motion their plan to halt the train.
First of all, they needed tools to loosen the rail. Being a remote location, the people in charge of the railroad tool shed were undoubtedly very happy to feed two strangers who showed up hungry, and talkative, perhaps sharing news and discussion of the hot July weather. While the locals were suitably distracted, the rest of the gang slipped into the shed and made off with tools to pull the spikes and dislodge the rail.
Get tools. Check.
Now, where to pull off the heist. The highest ridge in the area runs north to south through Adair. Runoff to the west of this ridge runs into the Missouri River, and everything to the right runs into the Mississippi. When the train crossed Turkey Creek, it headed up the hill to cross this ridge, making this the last significant 'uphill' section of track for many miles. At the same time, the train headed into a curve. The uphill curve slowed the train significantly.
Derail train. Check. Collect large quantities of gold. Wrong train. Bummer.
The last piece of the puzzle was escape. Adair is about halfway between Omaha and Des Moines, and at that time, a lightly populated area. It would take a while for the law to get organized to chase. Adair is also due north of the best place for the gang to lay low, which was northwest Missouri, an area with lots of friends and relatives and hidey holes.
Posse evaded. Check.
This looks like an accident waiting to happen.
The actual rails Jesse James loosened to derail a train.
Why It's Interesting
On July 21, 1873, Jesse James and his cronies came up with a new way to rob a train. They would derail it. They disconnected the rails, tied a rope to the loose rail, and hid behind a hill. When the train came by they yanked on the rope pulling the rail out of alignment causing the train to crash. The train they robbed was supposed to have $75,000 in gold, but to their dismay that shipment had been delayed. The gang only found $2000 in the safe, and what they could plunder from the passengers.
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