By Rob Cameron
BBC News, Wyoming
Coal helps power the US economy
The United States is the world's most powerful economy, but much of that power is derived from rather old-fashioned sources.
More than half of the country's electricity is produced by burning coal, and as demand for energy increases, so does the pressure on those who supply it.
Mention to the average American that you're going to Wyoming, and they are as likely as anything to ask you why.
It is a fair question. Wyoming consists mostly of scrubby grassland or arid, tan-coloured mountains, and with just 500,000 people, it is the least populous state in the US.
But the people of America should be thankful to Wyoming, because its colossal treasure trove of natural resources is helping - literally - to power the US economy.
Wyoming's minerals include crude oil, natural gas, uranium, methane and something called trona.
Don't worry if you haven't heard of it - few have. Trona is used in the manufacture of glass, and Wyoming has more of the stuff than anywhere else in the world.
But most of all, Wyoming has coal. Huge, thick, multi-layered seams of coal lie just a few metres below the surface.
Most of it lies in the Powder River Coal Basin, that spans the border with Montana.
And the Powder River Basin is providing America with a staggering one million tonnes of coal each day - about a quarter of all US coal production.
"It all boils down to what's cheap and reliable," says Lee Terry, Republican congressman for the town of Omaha, in neighbouring Nebraska.
"That reliability means cost. Coal is very cheap, and so you're going to see a continued reliance."
Wyoming's mineral-rich landscape is anything but empty
Cheap it is, no doubt about that. Cheaper than natural gas, which is why coal is used to generate 52% of America's electricity.
It is also plentiful. The United States contains the largest coal reserves in the world, enough to last for 250 years or more.
But the energy utilities that burn the coal take issue with the reliability.
They complain that supplies are not meeting demand, leaving them low on reserves and Americans potentially exposed to power shortages.
In a country where most homes, offices and restaurants are heavily air-conditioned, this is clearly a problem.
So who is at fault?
The railroads - in this case US's largest, Union Pacific - play a crucial role in the coal supply chain.
Union Pacific moves huge quantities of coal from the Powder River Basin to the power stations of the east. The task is approached with steely determination by men and women who clearly take deep pride in their jobs.
Locomotives move huge quantities of coal to US power stations
"It's the 1,000lb gorilla that you have to watch day in day out," says Cameron Scott, Union Pacific's general superintendent of railway operations, in North Platte, Nebraska - a crucial transport hub for US rail freight.
"The coal trains represent about 75% of everything that runs in and out of this yard."
And coal trains are no ordinary trains.
They typically consist of up to 135 cars - making them about a mile and half long.
Getting the coal to its destination is therefore a massive logistical challenge.
Union Pacific's space-age Harriman Dispatching Center, in Omaha, uses satellite-based GPS and other state-of-the-art technology to keep track of its trains every minute of the day.
Some, however, are beginning to question the wisdom of such heavy reliance on coal.
Powder River Coal is described as "clean" coal - that is, low in sulphur.
But critics believe it will never be feasible to burn coal on such a large scale without doing some damage to the environment.
The answer, they say, is a proper commitment to finding alternatives to fossil fuels. Burning coal is essentially a 19th-Century solution to a 21st-Century problem.
"What people forget is in the 1960s, in order to win the space race, the US government put about 5% of the federal budget towards the space programme," says Jim Esch, the Democrat candidate standing against Lee Terry in Omaha.
"I think we can do whatever we want with energy. If we want to come up with brand new forms of energy, I'm pretty confident we can do that too."