Here is a passage from the book Walt Disney Imagineering. It is a narrative account of your wild journey on Big Thunder Mountain.
The Miner Details of Big Thunder
Big Thunder Mountain dates back to the wild and woolly boom town days when every prospector west of the Rockies was looking for gold. The following is the tall tale heard tell by one of those prospectors who got it second-hand from old Sam, the last of the Big Thunder Miners:
Yessir, it is 1840, and around these parts, things got prit’ near quiet as the hangin’ tree on Sunday after the Big Thunder Mine tapped out. One day there ain’t none richer, the next, even a ghost wouldn’t have much innerst in her.
Things got mighty busted up and rusted down inside Big Thunder, so Sam told me while sluggin’ from a dusty bottle of Old Imagineer. He was the last prospector inside that mine. Fact is, poor old Sam took a spill and done landed belly up in one of them ore cars. Next thing he knows, the car takes off like a skinny coyote after a plump hen!
Off he went, a headin’ fer the mine. Seems like that old ghost mine came to life for Sam. He swears the rusted winch engine was a pumpin’ and a wheezin’ and just when he was thinkin’ he must have bats in his belfry, there was bats! Then he sat up to see what he could see in the dark, and there was pools of rainbow water and waterfalls, and plenty of them rocks the schoolmarm calls "stalactites and stalagmites".
The walls of the canyon kept comin’ in closer and closer at old Sam and he yelled until he couldn’t yell no more. All of a sudden, the car thunders into a pitch dark tunnel, with Sam holdin’ on fer dear life. Comin’ back out the other side, he spots a couple a danged skunks foolin’ with blastin’ powder, like to blow the top off a whole derned mountain! Little ways away, danged if’n there ain’t a Billy goat chawin’ on a stick of the stuff! But Sam didn’t have no time to worry about that, ‘cuz next thing he knows he’s whippin’ down Spiral Butte and headin’ right back down into Big Thunder Mine. Sam figgered he was goin’ in and never comin’ out this time, with all that rumblin’ and shakin’ and rocks comin’ down all around him. He closed his eyes tight but the next thing ya know he was outside and high-ballin’ down on the track again, right over the Bear River Trestle Bridge.
That ore car finally squealed to a stop right smack dab in the middle of Big Thunder Town. Sam just sat up, brushed off the dust and said, “I ain’t had this much of a whoop and a holler since the Grub Gang hit town. I just barely got out with my hide!”
Sam’s amazing ghost story was told and retold over the years, and because of it, no one was ever brave enough to even set foot near the mine – until the day a bold young Imagineer heard the tale and thought it might be fun to take a ride on old Big Thunder himself. Sure enough, he did, and the train turned out to be so much fun he decided to officially re-open the mine. Folks soon heard the news about Big Thunder and began to arrive there to take their own wild ride on the legendary runaway train.
Big Thunder Mountain is intriguing as the backstories vary depending on which version of the ride you're visiting, and has frequently been written and rewritten. Perhaps its the mischeveous miner ghosts killed in an avalanche, or perhaps its an Indian curse angered by the descration of the sacred mountain (and if so, does vengeance come in the form of earthquakes or flash floods). Disneyland Paris has a completely seperate backstory weaving all elements of the Frontierland town there, Thunder Mesa.
This story below comes from the Spring 1992 issue of Disney News, written at a time when Tony Baxter's Discovery Bay concept was going to have been built north of Big Thunder, and the northern edge of the Rivers of America, where Big Thunder Ranch now sits.
Discovery Bay and Big Thunder Mountain
The highly imaginative tale includes the legend of a young inventor, named Jason Chandler, who lived in a town called International Village during the peak gold rush years in the Big Thunder region - circa 1849. According to the chronicles, "...the young inventor devised a drilling machine with the capability of boring into the very heart of Big Thunder Mountain. There, the veins of gold ran so deep, it was rumored they could produce a mother lode that would bring a man enough wealth to last a hundred lifetimes and more."
But a cave-in occurred on Big Thunder, burying 26 miners alive. They would have drawn their last breath then and there, had it not been for the inventor and his laughable drilling machine. He burrowed down into the Earth’s core, rescuing the miners from certain death. It should have been a moment of joy and celebration, but as the men scrambled to the arms of safety, a massive earthquake shook the ground and a cavernous maw opened up, swallowing the inventor and his machine whole."
The miners, as well as the citizens of the village, struggled day and night against the mountain, trying to dig the young man from his living tomb. But they never saw him, or another nugget of gold, again. Big Thunder had taken its vengeance not only on the miners, but on their wealth as well. The mountain had gone bust, and it became just a matter of time before only ghosts resided there.
The official SIG (Show Information Guide) for Big Thunder Mountain has another story attributed to Tony Baxter, and also appears in Jason Surell's book The Disney Mountains. Of all the versions, this appears to be the most canon.
Big Thunder Mountain Ghost Story
In the Disney version, gold was discovered in Big Thunder country in the 1850s, shortly after the Gold Rush began near John A. Sutter's Mill in California, leading to the formation of the BTM Mining Company. But the locals believed Big Thunder Mountain and the land around it to be sacred, and a protective supernatural force dwelt deep within the mountain to protect it from anyone who might deface it in the pursuit of profit. At first, the mining operation went along without incident, but as the miners began using explosives to blast deeper and deeper into the unforgiving rock and laying tracks for the mine train they'd use to retrieve its golden bounty, the mountain's ancient fury was unleashed. Strange noises emanated from a newly opened mineshaft. The spirits of long-dead miners could be heard tapping on the boarded walls of abandoned tunnels. Cave-ins became common occurrences. And then the narrow-gauge engines began rolling out of the station with no human hands at the controls. Entire trains, most times packed with unsuspecting passengers, would race driverless, at breakneck speed, along the spiraling steel and wooden track. The miners began to concede that perhaps the locals were right all along. Maybe the mountain - and their mine - was cursed. They abandoned their posts, the BTM Mining Company went bust, and soon Big Thunder became just another ghost town dotting the Old West.
To make things even more complicated, the name of the town Big Thunder is located in seems to change - is it Rainbow Ridge, Tumbleweed, Thunder Mesa or Big Thunder?! It's certainly intriguing reading through all of these. For a detailed article on the Big Thunder backstories, take a look at a post at the WDW Radio Blog.