This is India's Darjeeling Himalayan Railway.

From the air, the train's journey across the Himalayas looks truly dramatic.

With UNESCO World Heritage status, the 140-year-old railway is maintained by a whole army of engineers.

Filmed from above, this series showcases some of the most incredible railways on Earth.

We'll gain a unique insight into these remarkable trains and the extraordinary landscapes they pass through, revealing the technology that built them, and the dedicated people who keep them running.

We'll uncover the culture, the history, and the engineering that makes these Truly Epic Train Journeys From Above.

India is one of the largest countries in the world.

With 1.4 billion people, it's home to one sixth of the world's population.

The Indian Peninsula reaches south into the Indian Ocean.

And to the north, it's separated from mainland Asia by the world's youngest, yet highest, mountain range... ...the mighty Himalayas.

(wind gusting) As the first rays of sun kiss the snow-clad peaks of the Himalayas, they illuminate a spectacular landscape.

In the lap of the world's third-highest mountain, Kangchenjunga, this is the hill town of Darjeeling.

It's famous for two things, the world-class black tea and the steam train that once took the tea to the world, the Darjeeling Himalayan Railway, better known simply as the DHR.

The DHR connects the mainline rail hub of New Jalpaiguri in North Eastern India, to Darjeeling, 88 kilometres further north, in the Himalayan foothills.

Every morning, the stations at each end of the line see two very different trains begin their journeys.

Here, at Darjeeling's distinctive station, a steam locomotive is on its way to link up with two carriages, ready for what's called the 'joyride', a tough, uphill, six-kilometre journey to the highest point on the route.

It's being driven today by Dilip Sinchury, one of the DHR's most experienced locomotive drivers.

Down in the plains at New Jalpaiguri, alongside the mainline locomotives, a small DHR diesel engine is preparing for a much longer journey over the whole 88-kilometre route, up the mountains to Darjeeling.

(train whistle blares) The journey will take the diesel train over seven hours, at an average speed of less than 15 kilometres per hour.

The route ascends 2,000 metres, That's a massive two kilometres up huge gradients.

It's only able to achieve this thanks to some extraordinary engineering en route.

On its way up the mountain, the train travels over 554 bridges, around nearly 900 bends, and crisscross the adjoining highway over 100 times.

It also passes through five distinct climatic zones, from tropical plains, through temperate forest, up into the extreme weather of the alpine regions.

Diesel locomotives operate the entire route, from New Jalpaiguri to Darjeeling.

The specially-built small locomotives, the NDM-6s, are unique.

(train horn blasts) They were introduced in the 1990s, and soon became the workhorse of the DHR.

(steam hissing) But steam locomotives are the real superstars of the DHR.

(train whistle blows) (train chugging) As the steam train departs Darjeeling, from above, the whole town seems to be at a standstill.

Traffic is at a halt.

Locals stop in their tracks... ...and tourists pull out their cameras.

With its cool climate, Darjeeling became an increasingly popular resort in the late 19th and early 20th century.

(train whistle blowing) Now it's the bustling home to over 130,000 people.

From the vistadome coaches, passengers get great views of the town.

With a steam locomotive, preparations start early.

Today, just after dawn.

(steam hissing) Driving steam trains is in Dilip Sinchury's blood.

Dilip has come in early for a thorough check-up of the iconic B-class locomotive he's due to drive today.

It's over 100 years old.

Engines like this aren't made anymore, and neither are spare parts.

It takes all the experience and know-how of Dilip, and a whole team of mechanics, to keep these last remaining locomotives running.

This morning, they're at work addressing an issue on the locomotive.

It's on the side rods that connect the wheels.

In a steam locomotive, it's the side rods that transfer power from the piston to the driving wheels.

Brass holders are used to couple the side rods together, and over time, the inside of the brass holders wear down, and the side rods can become loose, as seems to have happened here.

(steam hissing) When side rods loosen, it causes severe vibration, and reduces the power transmitted to the wheels.

As Dilip predicted, the holder has deformed.

It's the cause of the problem.

On the first journey today, the train has to climb over 183 metres to the highest point, Ghum.

It needs to deliver peak power.

With passengers gathering at the station, the train has to depart on time.

The fitters have a solution.

A piece of soft metal is pounded into the deformed brass holder, giving it a tighter hold on the side rods.

With the fix complete, the brass holder is hammered back into place, reconnecting the side rods.

(tense music) (flames crackling) (hissing and whistling) The steam locomotive is now back to full power.

Dilip and the DHR start their day on schedule.

(train clattering on tracks) (whistle blowing) The train passes right through the centre of Darjeeling.

The track here serves as a gathering place for locals and tourists alike.

Luckily, the train rarely exceeds a speed of 15 kilometres per hour.

(train whistle blowing) Today, alongside Dilip, an apprentice driver, Jitu Jenna, is learning the route.

They're about to face their first big challenge, as they head into the steepest part of the line.

Down in the plain, 88 kilometres away from Darjeeling, the diesel train has begun its journey up the mountains.

The DHR may be 140 years old, but one part of it is brand-new... the air-conditioned coaches.

Introduced in 2018, they give passengers a comfortable view of the route.

But the AC coaches are heavy, and only the NDM-6 diesels can haul them.

As the train arrives at its first stop, one passenger has a special reason to look forward to the trip.

Historian Sushil Kumar Dikshit takes the train each year, in memory of his father, who'd been the DHR's permanent way inspector, responsible for the tracks.

Today, Sushil will be visiting a section of the railway redesigned by his father.

(Sushil) During my school days, 1956 to 1964, I rode every year the whole stretch, you know, for all the ten years or something like that.

Affectionately, it was called Miss Steam.

(train horn blares) In the 1880s, when the railway was built, engineers decided to squeeze the line alongside the existing road for horse-drawn carts.

But its curves and corners were too tight for trains running on standard four-foot, 8.5-inch gauge rails.

Their solution was to set the rails just two feet apart, among the narrowest gauges in the world.

To help trains take the corners safely, the locomotives and carriages were engineered with low centres of gravity.

140 years later, the route has barely changed.

There are no tunnels along the route, allowing passengers continuous, panoramic views of the Himalayas.

After its third stop, Sukna, the train faces a climb of more than 274 metres in just over eight kilometres.

With two fully-loaded coaches, there's a real risk of the wheels slipping.

This could cause serious damage to the surfaces of the rails, and of the wheel itself.

The DHR employs a simple solution, one that has stood the test of time.

(train horn blasting) (Sushil) There are certain steep portions, like one is to 22, steep grades are there, but to assist in the traction, the sandman is always there to pour the sand on the tracks, for extra adhesion.

Sanding the tracks is the main way to try to avoid the problem.

Additional sand can be spread by the train itself.

As the train climbs through the steeper sections, the sanders deploy.

The sand creates more friction between tracks and wheels, allowing the train to climb the sharp gradient without slipping.

(train whistle blowing) Sixty kilometres uphill, Dilip, and his apprentice Jitu, are preparing their steam locomotive for the toughest ascent on the way to Ghum, the highest station on the DHR.

But first, they need to stop to refuel.

The locomotive needs to take on 600 kilos of coal, and 1,800 litres of water.

(steam hissing) With the water tanks full, it's time to begin the climb.

(train whistle blasts) The apprentice driver, Jitu, has only recently been promoted.

Before that, he was a fireman.

It will be at least another year before he can take sole charge of a steam engine.

Jitu has to keep a watchful eye on both the water level and the steam pressure.

The locomotives work by creating high-pressure steam in their boilers, which drive pistons that turn the wheels.

If the steam pressure is too low, the train won't make the gradient.

(train whistle blasts) Dilip and Jitu are gearing up to negotiate one of the DHR's several engineering marvels... ...the famous Batasia double loop.

There's a steep 42-metre climb on this section of the route.

In the early 20th century, increased loads of freight, including the famous tea, made it more difficult for drivers to negotiate the single loop, so in 1919, engineers added a second loop.

From above, it's impressive.

The line loops through an underpass and then circles round again, to pass over itself.

It eases the gradient, but it's still a tough climb.

After the Batasia Loop, the steam train still has a 120-metre climb to Ghum.

And about 40 kilometres further south, the diesel train is also facing a challenge, as it prepares for one of the steepest ascents on the route.

(lively music) The steam train is on the outskirts of Darjeeling.

This area is home to 87 tea gardens, estates producing around ten million kilos of tea each year, 80% for export.

In the 1870s, increasing tea production led to the building of the railway, allowing for a speedier and more efficient means of transport than horse and cart.

The train is now approaching the end of its journey and needs to slow down.

The steam locomotives and the carriages on the DHR have independent manual brakes.

The two brakemen, one in each coach, and the driver, have to be able to apply, or release their brakes, simultaneously.

Without being able to see or talk to each other, the solution is a tried and tested system... ...whistles.

(train whistle blasts) (tranquil music) As the train nears its destination, Ghum, a pointsman gives the train the all clear to cross into the 19th century-station.

(car horn honks) (man) This train is definitely a charming one, because this is something you don't get anywhere else, there are only a few places.

It goes along with the road, and we can see the shops and other things, as though they're going along with us.

(train whistle blasts) In preparation for the return journey to Darjeeling, the steam locomotive has uncoupled from the carriages, and is reversing to join with the other end of the train.

It's the first of four return trips for Dilip and Jitu today.

Down the line, the diesel train is now two hours into its journey.

From above, the view is of a huge area of temperate forest.

The train's entering the southern end of the famed Mahananda Wildlife Sanctuary.

It's home to a wide range of birds and animals, including Indian elephants.

(tense music) Leaving the sanctuary, the terrain gets steeper.

The train must climb 250 metres in just six kilometres.

There was once a loop here, making the climb less steep, but in 1942 it was destroyed by a massive landslide.

With so much debris, reconstructing the loop was impossible.

A different solution had to be found.

(Sushil) My father's tenure working for DHR... ...he was assigned with the job to construct the Z reverse above Rongtong Station.

Sushil has his father's original sketch, showing how the damaged rail loop could be replaced by a new Z reverse.

On a Z reverse, a train moves from an uphill line it's travelling on, to a track higher up the mountainside, by reversing up a gradient linking the two.

Then, it continues its ascent on the higher track.

From above, the train seen moving forward on the first leg of the Z reverse, designed by Sushil's father.

(Sushil) There are several explanations about the Z reverse... ...but the most popular one being the ballroom dance, where once you get into a corner, you have to take two, three steps backward in order to go around the dance floor again.

Once the train can go no further, the points are switched so it can reverse uphill.

It's a challenging moment for the locomotive pilot.

(train horn blasts) (tense music) The train halts one more time for the points to be switched so it can move forward on the higher track.

(Sushil) Decades later, it stands as real as this sketch.

I would give credit to all the people, all the staff in those days.

All of them worked for 80 hours non-stop, to complete this damaged portion, by building a new Z reverse.

On the uphill journey to Darjeeling, the train will pass through five more Z reverses, and three loops.

But there have been warnings of a cyclone in the region.

(suspenseful music) Heavy rainfall, dense fog, and low visibility are set to make the driver's job even harder.

(lively music) Past the Z reverse, the train hits the very steepest section of the route.

It must climb 427 metres in just 12 kilometres... ...aided by double loops like the one engineers built here at Chunabhatti.

Seen from above, it's spectacular.

(train horn blasts) (train horn blasts) The train is now approaching Tindharia.

It's a railway town developed by the DHR as its main repair and maintenance hub.

Up here, the passengers can enjoy the cool subtropical winds of the region.

For over a century, the much-photographed B-class steam locomotives have been at the heart of Darjeeling's identity.

(wind gusting) The classic steam engines are rebuilt in this workshop.

It's perched right on the edge of a cliff.

In the early 1900s, the DHR had 34 of these engines.

They hauled 174,000 passengers and 47,000 tonnes of goods annually.

Today, only 14 remain in service.

As time took its toll on the rest, they were cannibalised for spare parts.

The Tindharia workshop has around 90 workers... (siren wailing) ...among them, the last mechanics with the know-how to restore the 100-year-old locomotives.

(motorbike engine thrumming) Uraj Pradhan is the chief engineer.

(engine off) There are currently two major restoration projects going on in the workshop under Uraj's supervision.

The first is of an iconic locomotive, number 782, the Mountaineer.

Built in 1899, it is one of the oldest engines of the DHR.

After more than 120 years at service, its restoration involves a complete boiler change.

(crackling) After the Mountaineer's old rusted copper boiler's removed, it joins two other century-old relics waiting to be scrapped.

The replacement boiler needs a new valve system, to control the timing and volume of high-pressure steam entering the pistons.

Today, one of the valves won't fit.

It has to be fixed.

It will be at least two months before the Mountaineer is back in working order.

But there's another engine that needs Uraj's more urgent attention.

(steam hissing) (indistinct conversation) He's been restoring a second steam locomotive for weeks, and today it's due to return to service.

At only 95 years old, the Iron Sherpa is amongst the youngest steam locomotives of the DHR.

But like the Mountaineer, it needed a new boiler, and a complete mechanical overhaul.

(indistinct conversations) (steam hissing) Today, the team is firing up the Iron Sherpa for the first time in three months.

The director of the Darjeeling Himalayan Railway is due to inspect the restoration, and hopefully give it the green light to return to service.

But inevitably, with a locomotive this old, there are challenges.

Today, there is a problem in the pressure regulator on a valve.

While Uraj and his team race to fix the steam valve... ...just a few kilometres away the crew of the diesel locomotive is facing a serious problem on the line.

It threatens to stop the train altogether.

(dramatic music) Past Tindharia, as the diesel train moves higher into the mountains, it enters a treacherous section on the route.

There are hills on one side, deep valleys on the other.

(train horn blasts) (suspenseful music) About ten kilometres ahead, there's a problem on the track.

(train horn blaring) Heavy rains have been lashing the area over the past two days.

(car horns honking) There's been a mudslide at Gayabari, blocking the track with mud and rock debris.

A maintenance team has arrived to carry out repairs.

The whole area is prone to earthquakes and landslides.

So as well as the emergency repairs, engineering works are underway to protect the track.

Today, senior managers from Indian Railways and the DHR are here to inspect the work.

The metal fasteners that attach the rails to the sleepers are known as ERCs, elastic rail clips.

Some of these have come loose, or even detached completely.

The rain has also shifted the ground, so in some sections, one rail is lower than the other.

The problems need to be put right before the train gets here.

From above, the threat mountain debris poses to the railway is clear to see.

(tense music) The diesel train is making slow but steady progress up the hill towards where they're working.

(tense music continues) Another major landslide in the same spot occurred just a few months earlier, washing away part of the track.

Services were disrupted for several months.

Aerial cameras show the massive embankment now holding up the tracks.

Despite all the reinforcements, the Paglajhora section continues to be a concern.

With the mud and boulders cleared and the fasters reattached, final checks are made on the alignment of the rails.

(indistinct conversations) The engineers finally give the all clear for the train to pass.

(tranquil music) With his track inspection complete, the next stop for the DHR director is Tindharia's locomotive workshop.

(indistinct conversations) The team there has less than an hour left to get the Iron Sherpa in working order.

(dramatic music) The engineers have been working for two hours to fix the steam regulator on the Iron Sherpa.

But Uraj has a solution.

He decides to add extra insulation to the screw thread.

It should make a tight seal and ensure there's no further steam leakage.

(tranquil music) It's a simple and effective solution.

Finally, they're ready for the director's arrival.

(train whistle blowing) Over its nine decades, the Iron Sherpa has seen India's independence and the modernisation of the railway.

(train chugging) (uplifting music) Now, thanks to the dedication of Uraj and his team, it has years more service ahead of it.

(train whistle blowing) (train chugging) It's evening, and the steam train and the diesel train are both approaching the Batasia Loop... ...one after the other.

(tranquil music) (train whistle blasts) Dilip Sinchury and his apprentice, Jitu, are returning after their fourth trip to Ghum.

From above, the two trains on the Batasia Loop make a spectacular sight.

(train whistle blowing) As dusk sets in, both the trains are approaching Darjeeling Station.

(metallic squeaking) En route is this picturesque Buddhist monastery, where people continue to pray even as trains pass by.

(train whistle blasts) (train chugging) First to reach the town, is the steam train.

Not long after, the diesel train arrives.

(car horns honking) (train whistle blasts) Dilip pilots the locomotive back to the shed... ...where final checks are carried out by apprentice Jitu.

After a successful day under Dilip's supervision, Jitu damps down the fire.

It's not doused.

Firing up a steam engine from coal takes more than five hours.

With more experience, Jitu will eventually take over from Dilip.

(uplifting music) (train whistle blasting) Tomorrow, the steam and diesel engines will once again tackle this challenging, but special route.

The incredible views of towering mountains... ...the engineering skills of those who keep it running against the odds, together with the railway's long history and its UNESCO status... ...all combine to make the DHR one of the World's Greatest Train Journeys From Above.