Destination: top of the world: find out what it takes to build a railway across the highest ground in the world.

Last July, a shiny train chugged along treacherous terrain toward the Tibetan plateau. Called the "roof of the world," this rocky mound is Earth's highest geological structure. It is also the world's biggest plateau--covering an area four times the size of Texas. Until now, world travelers would have had to spend days hopping from large airliner to small airplane to bus to reach this isolated region. Last July, the Qinghai-Tibet (Ching-high Tih-bet) railway--which extends from the city of Xining (Sh-urn-ing) in western China and crosses over a portion of the plateau before reaching the Tibetan capital of Lhasa--opened for service (see map, 15).

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The Chinese government had envisioned this lofty railway since the 1950s. But the challenges of building a train line at such a high altitude, they thought, would stop engineers in their tracks. So, to many, the new 1,956 kilometer (1,215 mile)-long railway represents an engineering marvel.

BREATH SAVER

Eighty percent of the railway sits at an elevation, or height above sea level, of more than 4,000 meters (13,123 feet). There, rail engineers had to bear the brunt of subzero temperatures and a thin atmosphere. In low-lying regions, air molecules get squashed together by the weight of the air above. But as you climb higher, this atmospheric pressure decreases, and the gas molecules spread out. Along most of the railway, every breath you take would contain roughly 50 percent less oxygen than it would in low-lying areas.

The oxygen-scarce air meant breathless conditions for the railway-construction workers. To prevent the dizzying headaches and nausea associated with high-altitude sickness, the train builders wore oxygen masks and spent two hours a day within one of the 17 oxygen-filled tents set up along the rail route.

FROSTY EARTH

The greatest construction challenge came from the large expanses of permafrost, or permanently frozen soft, beneath the tracks. "Anything built on top of permafrost requires special engineering adaptations," explains Oliver Frauenfeld, a climate scientist at the University of Colorado-Boulder's National Snow and Ice Data Center. Why? The pressure from passing trains generates heat that could cause the layer of frozen earth to thaw. So too could the pounding, as workers fasten the tracks to the ground.

Just like when an ice cube melts and leaves behind a lower volume of water, if ice crystals within the soil were to thaw, the soil would take up less space. That would cause the soft to sink. The result could be disastrous for a railroad anchored on top. With sinking soil, parts of the track could buckle like an accordion. That could unhinge even the sturdiest of tracks.

STAY COOL

Putting a stop to ground thawing proved tricky. "It is easy to say and hard to do," says Tingjun Zhang, a permafrost expert at the University of Colorado-Boulder's National Snow and Ice Data Center, who worked with the railway designers.

Zhang was particularly concerned about a length of track running over a section of the Tibetan plateau. There, the permafrost is called "warm permafrost," meaning that the layer is -1[degrees]C to -2[degrees]C (30[degrees]F to 28[degrees]F). That's just below freezing, so even a slight warming could tip this layer from rigid permafrost to soggy soil.

The solution: keeping the permafrost that's underneath long sections of the railway chilly. Check out the diagram (above) for some of the "cool" technologies that should keep this train on the right track.

LESON PLANS

EARTH: Permafrost

Destination: Tap of the World

PRE-READING PROMPTS

Jump-start your lesson with these pre-reading questions:

* The Qinghai-Tibet railway is the world's highest railway, reaching an elevation of 5,072 meters (16,640 feet) at its highest point. What do you think are some of the challenges in building a railroad at such a high altitude?

* The train is designed to reach a maximum speed of 160 kilometers (99.4 miles) per hour. Hundreds of kilometers of the tracks run over permanently frozen earth called permafrost. What might happen if the earth thawed?

* Suppose you want to make sure the permafrost under the train tracks doesn't thaw. What are some ideas for keeping the ground chilled? Be imaginative!

CRITICAL THINKING

* Many endangered animals, such as the Tibetan antelope, live nearby the Qinghai-Tibet railway. How do you think the railway will affect the animals? What are some ways that conservation scientists could help to protect the wildlife?

CROSS-CURRICULAR CONNECTIONS

GEOGRAPHY: This Web site contains descriptions of train stops along the Qinghai-Tibet railway:

www.travelchinaguide.com/cityguides/tibet/train-to-tibet /scenes-intro.htm

Have each student select a stop and research his or her chosen location. Then have each student write a travel journal entry, describing a day visiting the area. Conclude by having each student share his or her adventure with the rest of the class.

RESOURCES

* For news and background information on the Qinghai-Tibet railway, including a map of the train route and related photos, visit: http:lltibet.cn/en/newfeature/qtrailway/index.htm

* For an informative article with great photos on the construction of the Qinghai-Tibet railway, read "Train to the Roof of the World," by David Wolman, WIRED, July 2006. Article also available online at: www.wired.com/wired/archive/14.07/chinarail.html

* Test students' math skills by having them look at a detailed schedule of the Qinghai-Tibet railway and calculate the time between different stops. This site contains a train schedule: www.chinahighlights.com/qinghai-tibet-railway/trains-schedules.html#b

CHECK FOR UNDERSTANDING

DIRECTIONS: On a separate piece of paper, defend or dispute the following. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

1. The Tibetan plateau is called the giant belly of the world.

2. Due to the low elevation, builders of the Qinghai-Tibet railway breathed in so much nitrogen that they suffered from altitude sickness such as headaches and nausea.

3. The greatest challenge in building the Qinghai-Tibet railway came from the permafrost beneath the tracks.

ANSWERS

Destination: Top of the World

1. Dispute: The Tibetan plateau is called the roof of the world because this rocky mound is Earth's highest geological structure. It is also the world's biggest plateau, covering an area four times the size of Texas.

2. Dispute: In low-lying regions, air molecules get squashed together by the weight of the air above. But as you climb higher, this atmospheric pressure decreases, and the gas molecules spread out. Along most of the high-altitude railway, every breath you take would contain roughly 50 percent less oxygen than it would in low-lying areas. This oxygen-scarce air meant breathless conditions. So builders of the Qinghai-Tibet railway suffered from high-altitude sickness such as headaches and nausea.

3. Defend: Anything built on top of permafrost, including the Qinghai-Tibet railway, requires special engineering adaptations. The heat generated by the pounding needed to fasten tracks to the ground and the pressure from passing trains could cause the layer of frozen earth to thaw. Just like when an ice cube melts and leaves behind a lower volume of water, if ice crystals within the soil were to thaw, the soil would take up less space. That would cause the soil to sink. The result could be disastrous for a railroad anchored on top. With sinking soil, parts of the track could buckle like an accordion. That could unhinge even the sturdiest of tracks.