The Assassin (Sha ren zhe Tang Zhan)

This time on "Impossible Engineering,"

the highest railway ever built...

The engineering is mind-blowing.

It was almost impossible

to conceive of a railway running at this altitude.

Constructed in a remote, frozen wilderness...

It's definitely amazing.

It's a totally different setting.

And the pioneering, historic innovations...

We're at 30 meters.

One of the biggest changes is my voice.

I speak in a very strange manner.

I can sense the lack of oxygen through my lungs.

Lightheaded. Do not feel great.

That made the impossible possible.

captions paid for by discovery communications

this is Lhasa,

the spiritual home of Tibetan Buddhism.

This city in the clouds is one of the highest,

most inaccessible on earth.

Situated on the vast Tibetan plateau,

it is surrounded by not one

but three mighty mountain ranges.

Before the 1950s, there weren't even any roads

to get to Lhasa... Just meandering mountain paths

that took months to get through, and only in good weather.

But the Chinese government came up with an audacious scheme

to connect this ancient province with modern China.

This is the highest railroad on the planet.

Specially-designed, high-altitude trains

climb to 16,400 feet above sea level

to cross the Tibetan plateau...

The highest and largest mountain plateau on earth.

Our locomotives can pull 3,700 tons of cargo

or 20 passenger carriages.

It's the most advanced technology in our country.

The engineering is extraordinary.

675 Bridges and 10 tunnels

to cross the frozen wilderness

that's known as "the roof of the world."

Whenever I see the train passing through the Tibetan plateau,

I feel extremely proud.

From Beijing and Shanghai in the east,

trains run to the city of Golmud,

the start of the high-altitude line.

From here, they begin their climb,

crossing the formerly impenetrable Kunlun mountains

to reach the immense, frozen Tibetan plateau.

At this altitude,

the air oxygen content is half that at sea level,

and temperatures can drop below negative 20 degrees.

The railroad reaches the world's highest station

at the Tanggula pass,

a staggering 16,600 feet above sea level,

before making its way to Lhasa.

This is the Qinghai-Tibet line,

a railway which at first people thought was impossible.

It's called the heaven road,

and, honestly, that's what it is.

It's the most audacious railway project ever imagined,

so it comes with some enormous challenges.

Perhaps the biggest challenge

is how do you keep everyone breathing?

At this incredible altitude,

oxygen is in short supply.

Dr. Zhu Xinxiang is in charge

of the railway's medical unit.

On the Qinghai-Tibet railway

between Golmud and Lhasa,

the average altitude is over 4,000 meters.

It's an area of extremely low atmospheric pressure.

There's only around 50% oxygen in the air here

compared with the mainland.

The human body is not designed to cope

with such low oxygen levels.

It can lead to potentially fatal altitude sickness.

In severe cases where the body's starved of oxygen,

it can cause swelling of the brain or lungs,

which can be fatal.

These unique trains are specially engineered

to protect their passengers

as they travel through this beautiful but brutal landscape.

Ding Weiran is the senior engineer.

In here is the oxygen generator,

which pumps oxygen into the train carriages.

These prevent oxygen shortages at high altitude

and make passengers feel more comfortable.

Inside, there is an altitude sensor to determine

the oxygen levels outside, and oxygen concentrations sensors

to monitor the levels in each passenger compartment.

The train is made up of 16 carriages,

and each of these carriages

has its own oxygen generator like this.

The passengers also have access to oxygen masks,

which they can use if they're having difficulties.

Well, if you're just sitting here, then it's all right,

but if I climb to the top of the bunk bed,

then I notice that I have some shortness of breath.

As an extra precaution,

the railroad posts a medical team on every train.

These extra measures onboard ensure safe passage

for the 2 million travelers who make the journey every year.

But the team that built the line didn't have these luxuries.

Construction workers are at a high risk

of altitude sickness and hypoxia...

A severe lack of oxygen. Doing physical work

means your body uses up oxygen very quickly.

That made this an extremely difficult project.

At first, we thought there was no way it could be completed.

They would need to turn to a machine from the past

to defy nature and build across the roof of the world.

Dr. Gary Smerdon is in southwest England

at the site of Isambard Brunel's masterpiece,

discovering how experts in engineering and medicine

came together to solve the problem.

So, here we have the royal Albert bridge,

a fantastic feat of engineering.

Most of the people who travel across the top of this bridge

do not have the faintest idea about the effort

and the suffering that went into building this bridge,

'cause the feat of engineering to get that pier built

down into the rock bed of the river

was very new at the time, very dangerous,

and they didn't really understand what they were doing.

These workers were moving into a pressurized environment

every day to do their digging and their engineering,

then coming back to the surface.

There were 25 workers. One died, two became paraplegic,

and all of the others became ill

due to what became known as caisson disease.

With 19th-century engineers

wanting to build ever more impressive structures,

a way to treat workers who came down with caissons disease

had to be found.

In 1876, American inventor Daniel Kelly

submitted a patent in Chicago for a compressed-air bath,

a machine that would hold the key

to treating workers with caissons disease...

Today commonly referred to as decompression sickness

or the bends.

The body contains nitrogen gas.

When the pressure increases,

this gas dissolves into the blood and tissue.

But when the body returns to normal pressure,

it becomes a gas again.

If bubbles of nitrogen gas form, they can gather in joints

and tissues, causing serious problems.

Kelly's machine was a forerunner

of the hyperbaric chamber.

Any problems, just put your hand up,

stay "stop," and we'll take it from there.

Lovely. See you on the surface.

While the condition affecting the caisson laborers

was very different

from what the Chinese workers were suffering from,

this amazing machine holds the key

to solving both problems.

An air compressor creates pressurized air,

which is stored in a giant tank.

It's slowly pumped into the hyperbaric chamber,

recreating whatever pressure environment

doctors need to treat their patients.

This was the key to treating caissons disease,

giving doctors the ability to reduce

those deadly bubbles in the blood.

So, here we are. We're in the chamber now.

The door is shut.

My colleague, Chris, is at the controls,

and he'll soon be pressurizing me

and getting me down to the equivalent of 30 meters.

And the machine itself can actually be used

to demonstrate the very problem it was built to solve

with a simple experiment.

So, with me here in the chamber, we have an empty bottle here

that's full of air

and another bottle that's full of carbonated water.

The empty bottle contains air

at normal sea-level air pressure.

So as the pressure in the chamber mounts,

the bottle collapses.

But that's not the only apparent change

in the pressurized chamber.

Okay, that's you at 30 meters, Gary.

Okay.

Gary's vocal chords are feeling the pressure, too.

And there we have a glass

of carbonated water.

The water, which was carbonated at normal air pressure,

has completely lost its fizz.

That much pressure, the gas molecules

are dissolved in the liquid, but the gas is still in there.

This water represents blood in the body.

So when we go back to the surface, you will see,

bubbles will start to appear.

That's exactly what was happening

to these caisson workers,

where bubbles will start to appear in your blood

and in your tissues. As a human, you're in trouble.

It was discovered that the key

to keeping bubbles from forming