(CNN) -- A high-speed passenger train derails as it rounds a curve, killing dozens and injuring scores more.
That scene played out Wednesday in Spain, leading U.S. residents to ask, "Could it happen here?"
The answer: Not likely, not now, if only because there's precious little high-speed rail in the United States. Also, the train that crashed near Santiago de Compostela is an extremely complicated machine, nothing like any train in the United States, said University of Dayton assistant professor Steven Harrod.
"From the U.S. public's point of view, I think it's important for them not to be scared or panicked about this," said Harrod, whose research expertise lies in railway operations. "I could never imagine such a complicated piece of equipment running in North America."
Where the United States, like most of the world, has a standard track width of 4 feet, 8.5 inches, the Spain train was designed to run on both that gauge and on wider tracks that are used in much of Spain and Portugal, Harrod said.
"The wheels on this train actually move, so you can imagine the complexity that adds," he said.
Spain's high-speed AVE trains run on a different voltage than other trains in the country, so this train's electrical system was outfitted to adapt to multiple voltages and signal systems, and because parts of Northwest Spain have no electric wires, the train was designed with a diesel engine so it can run on both electric and nonelectric tracks.
"It's an enormously complicated train, nothing like the U.S. or North American continent has ever seen," Harrod said.
In the crash video, he said, "the train looks like it peels off the track between the diesel engine and the first coach." This is the weakest juncture in the train, he said, because the coach car is far lighter than the engine.
America's slower locomotives
While Harrod said there are routes between Chicago and St. Louis and Chicago and Detroit where trains can top 100 mph (the Spain train was reportedly running about 118 mph), the closest the United States has to high-speed rail is the Acela Express that runs between Washington and Boston at advertised speeds of up to 150 mph.
But that's a bit misleading, some say. The trains rarely travel that fast because along many stretches the tracks aren't rated for that kind of speed. Amtrak announced in September it plans to run test trains at 165 mph in Delaware, Maryland, New Jersey, Rhode Island and Massachusetts.
"They're operating on tracks and going through tunnels that are 100 years old," said Robert Puentes, a transportation expert at the Brookings Institution.
While Acela trains could potentially derail, Federal Railroad Administration requirements demand they be heavier than similar trains in Europe, making them less likely to fly off the tracks, according to an excerpt from "Waiting on a Train: the Embattled Future of Passenger Rail Service," posted on the publisher's website.
Speed limits enforced by the agency also keep the trains moving slower than their powerful engines allow, further reducing the risk of an accident.
Those limits are necessary because of curves, small tunnels and freight traffic along the same tracks, according to the book, written by James McCommons.
"The high-speed corridors in Europe and Japan are straight, level, sealed from intrusion, and set aside solely for fast passenger trains," McCommons writes. "To create such a corridor in the Northeast with its dense population would cost tens of billions of dollars and require the purchase or condemnation of more land and neighborhoods."
The curves, traffics and tight tunnels may actually make U.S. trains safer, Harrod said, because engineers have to pay more attention, especially since there's far less automation in American trains than in European ones.
"Human factors play a big role, including the risk of oversight due to boredom. The Northeast Corridor is curvy with frequent speed changes, so engineers there never have a chance to get complacent or bored. They are constantly braking and accelerating," he said. "We don't have this level of computer control. In most places in the U.S., the engineer's in the cab. He's watching visual signals outside the cab."
American trains also have an auto-stop feature, so if an engineer misses a stop or caution signal, either inside or outside the cab, the brakes are applied automatically.
Slow progress for high-speed rail
High costs and political opposition have led to limited success for efforts to bring widespread European- and Asian-style high-speed rail to the United States.
President Barack Obama used the 2009 economic stimulus package to dole out $8 billion for high-speed rail projects, and in 2011 proposed spending $53 billion over six years to promote construction of high-speed lines around the nation.
More than two years later, there's been little progress.
While the federal government offered money to spearhead high-speed rail efforts in states such as Wisconsin, Ohio, Florida and California, Republican governors in Wisconsin and Florida killed the projects.
In California, voters in 2008 approved a 520-mile high-speed rail project that would have carried riders between San Diego and San Francisco at nearly 200 mph. Since then, the cost estimate has gone up, speeds have come down and the route has been limited to a roughly 200-mile stretch between Fresno and Burbank.
Now, the attention appears to be turning to state and local initiatives.
A private company has proposed a line in Texas, while another private proposal would bring high-speed rail linking Miami and Orlando, Florida.
But for high-speed rail to make significant inroads into the United States, the nation would first have to make significant improvements to basic infrastructure, such as tracks and bridges to allow freight and passenger traffic to coexist, said Brookings' Puentes.
"We've got a long way to go," he said.
CNN's Thom Patterson contributed to this report.