The Assassin (Sha ren zhe Tang Zhan)

In this episode...

Crossing chasms...

Bridging nature's most challenging divides...

In winter, the water just chucks it down this valley

through almost impenetrable forests.

With the unique engineering solutions...

Engineers weren't gonna let earthquakes stop the railway.

That make the impossible possible.

captions paid for by discovery communications.

Many of the world's greatest railroads have defied nature,

overcoming its most difficult terrain.

Whether scaling sheer heights or navigating dense forest,

engineers have managed to carve out routes

to create the most epic lines imaginable.

But crossing chasms tests them to their limits.

From ferocious rivers

to remote, windswept valleys...

Uniquely engineered bridges

crucially keep the world connected.

But each of these crossings raises individual challenges

that are often seemingly impossible

for railroads to overcome.

Well, the key challenge is the tidal range.

The water flows in and out

of a quite constricted channel very fast.

But, arguably, the biggest obstacle

facing bridge engineers is a hidden one.

Located on the notorious ring of fire,

New Zealand's brooding volcanoes are a stark reminder

it sits squarely on a major fault line...

Where the Australian and pacific tectonic plates collide.

Here, the devastating effects of earthquakes

are an ever-present threat.

Heritage advisor Karen Astwood

has traveled into its rugged interior

to see how engineering played its part

in keeping a vital railroad safe from seismic shifts.

What I'm approaching now

is one of the north island main trunk original tunnels.

When the line was constructed in the early 1900s,

it became incredibly important

because it connected Auckland and Wellington,

which is the north island's two major cities.

But by the 1960s,

this particular section of the main trunk line

in the Rangitikei district was putting the route in jeopardy.

Many of the tunnels built were in danger of collapse.

Upgrading this section simply wasn't practical.

The unstable ground

meant the tunnels weren't feasible to strengthen,

and neither was creating new ones.

Instead, engineers came up

with an ambitious plan to reroute the original line,

known as the Mangaweka deviation.

But in the way lay what appeared to be

an insurmountable obstacle.

Here it comes. This is the South Rangitikei viaduct.

It is immense. What an amazing structure.

So impressive.

Opened in 1981

and measuring a staggering 1,030 feet in length,

the mammoth six-span viaduct

carries a single track across twin-legged piers,

a vertigo-inducing 250 feet above the river.

Wow.

But to see what makes this bridge truly revolutionary,

you need to look much closer to the ground.

When designing the South Rangitikei viaduct,

engineers had to consider the earthquake conditions

it needed to operate under to keep the critical north island

main trunk line functioning.

It was the groundbreaking work

of eminent earthquake scientist and engineer Dr. Ivan Skinner

which provided the answer.

At the time the Mangaweka deviation was being planned,

seismic engineering technology was in its infancy.

So the designers of the South Rangitikei viaduct

had to come up with a completely new solution...

Base isolation.

The first of its kind in the world,

the bridge's innovative design

features energy-absorbing dampers in the foundations,

which allow it to step from side to side when a tremor hits.

Okay, so, we're just putting together

a really basic demonstration

to give you an idea about how base isolation works.

To begin with, we've got a shake board,

which is going to mimic

the horizontal forces of an earthquake.

Now, usually, you'd build your bridge straight onto the earth.

But the South Rangitikei viaduct, however,

we've got the foundations,

and then we've got the base isolation,

then we've got the pier.

Unlike traditional Bridges,

the foundations consist of two sections...

One built into the ground

and the other fixed to the bottom of each pier.

At the base of each pier sits a set of rubber pads,

which act to absorb a portion of the energy

created in the event of an earthquake.

So, these tennis balls

are standing in for the flexible bearings

or pads that are in the base isolation.

And this is a platform that the bridge pier is gonna sit on.

Okay, so, now that we've got the foundation sorted out,

we're gonna build our piers.

This is just a standard old bridge...

Build it straight into the ground onto the foundations.

And here is a pier from the South Rangitikei viaduct.

But to show you the full effect of how the base isolation works,

I've just got to duck off and get some water.

Rather than rigidly fixing the bridge,

the base isolators effectively separate it

from the ground for greater flexibility.

Okay, so, here comes an earthquake.

And as you can see, the one straight into the ground

is absorbing all of the energy from the earthquake,

so it's more likely to fail and the bridge collapse.

While the South Rangitikei viaduct...

It's not absorbing as much of the earthquake forces,

so it's less likely to fail in the event of an earthquake.

Under most circumstances,

the bearing pads absorb enough force

to keep the bridge structurally intact,

but in a major earthquake,

the pier can lift up by as much as 5 inches,

allowing it to step from one leg to the other,

preventing a catastrophic collapse.

And that's the genius of base isolation.

Every day, Ivan Skinner's

inspired innovation enables trains to traverse the length

of New Zealand's rugged north island,

keeping the country moving

even when experiencing the most terrifying tremors.

This is an ingenious piece of engineering, and I love it.

But the ground doesn't have to quake

to present engineering challenges

to those audacious builders behind the world's

most challenging railroad projects.

Southern France's rugged Auvergne region

isn't the most obvious place to build a railroad.

But at the end of the 19th century,

transporting wine from the region's vineyards

to the capital of France became a priority.

Forming a natural blockade, however, was the massif central,

a sprawling landscape of imposing peaks,

deep gorges, and famously strong winds.

Historian Patricia Rochés is taking to the skies

and taking on the notorious turbulence...

Wow!

To get a bird's-eye view

of why plans to build the new line were stalling...

the immense Truyère river gorge.

To combat the elements and bridge the valley

would require a feat of engineering ingenuity...

The breathtaking Garabit viaduct.

At 1,850 feet long and 400 feet high,

upon its completion,

Garabit was the tallest and longest railroad bridge

the world had ever seen.

The iconic design of the Garabit viaduct

was the work of one of the 19th century's

most celebrated engineers, Gustave Eiffel.

It would take Eiffel's unique talents

to make Garabit viaduct not only possible,

but one of the most spectacular railroad Bridges in the world.

When France needed a bridge

to span the immense Truyère river gorge

and withstand its famous winds,

they turned to renowned engineer Gustave Eiffel.

Today, Eiffel's solution to withstanding the gusting winds

will be studied up close by the team

tasked with maintaining this mammoth structure.

The design is one that would go on to earn him the nickname

"the magician of iron."

Instead of thick, solid girders,

Eiffel used smaller, crisscrossing wrought-iron beams

with thousands of triangular gaps.

His inspired design dramatically reduces wind resistance

as it's buffeted by the powerful gusts at Garabit.

Despite its lightweight appearance,

the Garabit viaduct was designed to carry a 400-ton train

and built to last.

The 540-foot-wide arch was constructed from both sides,

as cranes at each end extended it, piece by piece,

until the two halves were joined.

Metal structures expert Francois Milien

is part of the fearless team responsible for ensuring

the bridge continues to stand the test of time.

Taking five weeks to complete,

each of the bridge's

crisscrossed beams and 600,000 rivets

are inspected for signs of wear.

Eiffel's little-known masterpiece

of railroad engineering

remains a stunning example of his signature style

that would later inspire a Parisian icon,

the Eiffel Tower.

The Truyère river gorge inspired Eiffel