It is a natural question, one that many people have raised before, and have raised again since Tuesday’s fatal Amtrak crash in Philadelphia: Should trains have seatbelts?

Transportation experts have asked that question, too, in studies that cite a mix of physics, engineering, human behavior, and economics – and a healthy dose of uncertainty about their conclusions. Ray LaHood, a former transportation secretary, said this week that the federal government should consider it again.

But the research done so far has generally found that seatbelts would do minimal good, that the harm they would do might be greater, and that the cost of installing them would be high. The National Transportation Safety Board has not put train railroad seatbelts on its "Most Wanted List" of safety improvements.

"It’s been asked frequently, and honestly, I remain uncertain, but the conclusion has always been that you can’t justify it," said Steven R. Ditmeyer, a former director of research and research development at the Federal Railroad Administration.

Experts say they do not know of any country that has rail passengers strapped in.

"Only on roller coasters," said Steven Harrod, a transportation safety expert and associate professor at the Technical University of Denmark.

Why would seatbelts not help in a crash like the one in Philadelphia?

In that case, they might have helped – it is very hard to know for certain – but that would not have been true in other accidents.

Tuesday’s derailment was unusual, in that some passenger coaches rolled onto their sides, causing injuries that might have been prevented by seatbelts. But it may also turn out that the fatal injuries came from people being crushed, or from outside objects like rails or poles spearing through the coaches – hazards that seatbelts would not address.

When a car on a highway collides with another vehicle, or hits a stationary obstacle like a tree, a big part of the danger is sudden, extreme deceleration. That is the main reason seatbelts are so effective. Every object in the car that isn’t restrained keeps moving forward at high speed. That includes humans, who can fly forward into steering wheels and through windshields, suffering potentially fatal impact injuries.

But a train is so massive, with so much momentum, that it almost never experiences that kind of deceleration. (One exception is when two trains collide, which is very rare.) Less extreme deceleration usually means less extreme impact injuries. A train hitting a car or truck at a grade crossing barely slows down at first.

Consider this: If a car flipped onto its side at more than 100 mph and skidded off the highway, how likely is it that the people inside would survive if they were not wearing seatbelts? That is essentially what happened to Amtrak Train 188 in Philadelphia, and 235 of 243 people aboard lived.

The last American rail accident to claim more lives occurred in 2008, when a commuter train and a freight train collided head-on in the Chatsworth section of Los Angeles, at a combined speed of more than 80 mph, killing 25 people. Even then, 195 passengers survived.

In recent years, cars and light trucks, which have mandatory seatbelts and airbags, have an occupant death rate per mile that is about 10 times as high as the rate for passenger trains, which have neither.

The collision risk to people aboard trains is so low that seatbelts "would not add a statistically relevant level of protection," Harrod said. "Seatbelts have value on automobiles because the risk of collision is really very high."

Even if seatbelts only prevent a few injuries and deaths, isn’t that worth it?

Not necessarily.

Even if it could be shown that seatbelts reduced harm, on balance, they might not survive the cost-benefit analysis the federal government is required to perform. Put simply, federal regulators cannot require any change that would cost millions of dollars for each injury prevented.

Just as important, it is not clear that seatbelts actually would produce a net reduction in harm. Studies have shown that lap belts, like those used on airliners, would actually increase serious spinal injuries in train crashes, as people’s upper bodies whipped forward.

Lap-and-shoulder belts, like those in cars and trucks, would be much better, but installing them would mean replacing the passenger seats. The seats used now have a lot of give to them, and designers say shoulder belts would only work on much stiffer seats.

For people who do not wear their seatbelts, or are walking around, stiffer seats would mean worse injuries if a crash sent them flying.

"Passengers not wearing seatbelts would become projectiles that would collide with the seat ahead of them," said a 2007 report by the Transportation Research Board of the National Academies. "If occupants in the forward seat were wearing the seatbelts, they would be thrown forward with the compounded weight of the non-seatbelted passengers who were projected forward from behind. Their injuries would be compounded."

A study published the same year by Britain’s Rail Safety and Standard’s Board reached a somewhat more positive verdict on seatbelts, but did not recommend them. "It was found that injury outcomes for passengers choosing to wear restraints were substantially improved," the study said. "However, there was a slight general worsening of injury outcomes for passengers choosing not to wear restraints as they impacted the modified (stiffened) seat."

So couldn’t passengers be required to wear the belts, as they are in cars and airplanes?

On airplanes, belts are only required during takeoff, landing, and turbulence – usually predictable, brief events.

Trains are often standing-room-only, and cash-strapped railroads are not likely to provide more service to make sure that everyone has a seat and a seatbelt. On commuter trains that make frequent stops, passengers buckling and unbuckling could slow things down.

And part of the appeal of train travel is the freedom to move around. Enforcing a seatbelt rule would be burdensome, and experts say passenger compliance would be low. "Riding a passenger train allows personal freedom to walk about, to stretch your legs, go to the cafe car, use the lavatory," Ditmeyer said. "That carries a lot of weight with people, including passengers and policymakers."