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Why Are There Crushed Stones Alongside Railroad Tracks?

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Why are there crushed stones alongside rail tracks?

David S. Rose:

This is a good question with an interesting answer. The crushed stones are what is known as ballast. Their purpose is to hold the wooden cross ties in place, which in turn hold the rails in place.

Think about the engineering challenge faced by running miles of narrow ribbons of steel track on top of the ground: they are subject to heat expansion and contraction, ground movement and vibration, precipitation buildup from rough weather, and weed and plant growth from underneath. Now keep in mind that while 99 percent of the time they are just sitting there unburdened, the remaining one percent of the time they are subject to moving loads as heavy as one million pounds (the weight of a Union Pacific Big Boy locomotive and its tender).

Put all this together, and you have yourself a really, really interesting problem that was first solved nearly 200 years ago, and hasn't been significantly improved since.

The answer is to start with the bare ground, and then build up a foundation to raise the track high enough so it won't get flooded. On top of the foundation, you deposit a load of crushed stone (the ballast). On top of the stone, you lay down (perpendicular to the direction of the track) a line of wooden beams on 19.5 inch centers, 8.5 feet long, 9 inches wide and 7 inches thick, weighing about 200 pounds ... 3249 of them per mile. You then continue to dump crushed stone all around the beams. The sharp edges of the stone make it difficult for them to slide over each other (in the way that smooth, round pebbles would), thus effectively locking them in place.

The beams are made of hardwood (usually oak or hickory), and impregnated with creosote for weather protection. In the U.S. we call them "cross ties" (or, colloquially, just "railroad ties"); in the UK they are known as "sleepers"; European Portuguese, "travessas"; Brazilian Portuguese, "dormentes"; Russian, шпала (read "shpala"); French "traverses." While 93 percent of ties in the U.S. are still made of wood, heavily trafficked modern rail lines are increasingly trying alternatives, including composite plastic, steel, and concrete.

Next, you bring in hot-rolled steel rails, historically 39' long in the U.S. (because they were carried to the site in 40' gondola cars), but increasingly now 78', and lay them on top of the ties, end to end. They used to be joined by bolting on an extra piece of steel (called a "fishplate") across the side of the joint, but today are usually continuously welded end-to-end.

It would seem that you could just nail them or bolt them down to the ties, but that won't work. The non-trivial movement caused by heat expansion and contraction along the length of the rail would cause it to break or buckle if any of it were fixed in place. So instead, the rails are attached to the sleepers by clips or anchors, which hold them down but allow them to move longitudinally as they expand or contract.

So there you have it: a centuries-old process that is extremely effective at facilitating the movement of people and material over thousands of miles ... even though nothing is permanently attached to the ground with a fixed connection!

The ballast distributes the load of the ties (which, in turn, bear the load of the train on the track, held by clips) across the foundation, allows for ground movement, thermal expansion and weight variance, allows rain and snow to drain through the track, and inhibit the growth of weeds and vegetation that would quickly take over the track.

This post originally appeared on Quora. Click here to view.

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Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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