I travelled through India by passenger train and saw a side few visitors experience

Bip, bip, beeeeep! When I exit Kolkata airport and sink into the back seat of my ride to the hotel, it's midnight. After an all-day journey from Australia my eyeballs feel like sandpaper and I'm ready to relax for a moment - but India's honk-happy drivers have other ideas. As they lean on their horns, communicating in their own type of Morse code, I burst out laughing. India, you're something else and I can't wait to reacquaint myself with you.

I've been to the world's most populous country only once before - to Ladakh in the Himalayas, where life in the stark high altitudes is blissful and serene. Now I'm in one of its biggest cities, located near the eastern border with Bangladesh, about to embark on a pilgrimage to some of the country's holiest sites.

Tour operator Explore Worldwide specialises in "authentic" small-group adventure experiences - and there's nothing more authentic than riding the rails on India's regular passenger services. Our journey will mostly involve long-distance trains on the network that moves up to 25 million passengers (almost the equivalent of Australia's population) daily. I think I'm up for this awfully big adventure - so bring it on.

I'm doing a big chunk of Explore's 15-day tour that winds its way north-west across the top of the country from Kolkata to Amritsar - the holiest city for Sikhs that sits near the border with Pakistan. The company says this kind of slower-paced longer journey fits into a trend for "radical sabbaticals" - where middle-aged travellers aren't waiting until retirement to tick off leisurely dream trips.

Sadly my sabbatical will be on the shorter side. I'm finishing up in Delhi, missing out on the last leg that includes taking the bone-rattling, tunnel-burrowing World Heritage-listed Toy Train to the vertiginous Shimla hill station and the chance to admire Amritsar's famed Golden Temple - two experiences that previous guests have gushed about as tour highlights. There's still, however, a whole lot of India for me to soak up.

All I know of Kolkata before my arrival is that Mother Teresa was based here, helping the poorest of the poor (in 2016, she was canonised as Saint Teresa of Calcutta). On my first morning, as I stand beside a traffic-choked road wondering how on earth I'll cross it, I also experience kindness.

"Come," says a local man, indicating I should follow him into the maelstrom. Indeed the traffic diverts around us like some sort of miracle and I start to enjoy my solo outing before meeting up with my fellow travellers. I amble to the Hooghly River, a distributary of the Ganges, and find myself at a ghat - wide steps leading down into the water where people are bathing and practising their sacred rituals. I later learn that Babu Ghat, built in 1830, is the city's second-oldest ghat.

It's impossible to be in a pulsating city like Kolkata and not collect a few new favourite memories: those sunbursts of technicolour at Mullick Ghat Flower Market (one of the country's oldest and busiest flower markets that sells garlands of marigolds, bunches of roses and chrysanthemums, and more), goats being shepherded along hectic roads, a stroll across the iconic Howrah Bridge (famously built without a single nut or bolt), and the soaring double-height interiors of the nostalgic College Street branch of Indian Coffee House - a place once patronised by intellectuals and artists.

We're so charmed by the city's classic yellow Ambassador taxis that we take a short, unnecessary ride in one of these dinky "kings of the road" (fast fading into history, thanks to new vehicle age-limit regulations and other factors) just to savour the experience, which includes watching the driver wrangle the spindly stick-shift. To round out our Kolkata transport adventures, we also board one of the colourfully painted buses.

Kolkata has charmed me like a cobra but it's time to take a train for an almost 500-kilometre journey to Bodh Gaya - the place where Buddha is said to have attained spiritual enlightenment. We ride the rails from late afternoon, arriving just before midnight in Bihar (one of the country's "dry" states where it's illegal to consume alcohol).

Our group whiled away the journey in either single side bunks or open four-bunk compartments, with a flimsy curtain that can be pulled for a little privacy from the corridor. Some of us share our compartments with strangers (a local woman begs for my lower bunk and, as I awkwardly twist, turn and flop into her upper slot, I understand why).

An attendant first delivers a cold tray with a boxed sandwich and packaged samosa (I'm reclining with eyes closed, so he alerts me to its arrival by loudly slapping the vinyl mattress). Later, a hot dinner tray - loaded with chicken, dhal, rice and veggies - is washed down with a masala soda. The dining is simple but more generous than expected.

In Bodh Gaya, we head to the World Heritage-listed Mahabodhi Temple Complex - home to a large Bodhi tree said to be a descendant of the original under which Buddha meditated and achieved nirvana - or enlightenment. Our visit coincides with a chanting festival - the grounds are packed with pilgrims who have settled in for the long haul, minimising distractions by sitting in small tents or donning eye masks.

For some of our group, it's a bummer to discover that no mobile phones are allowed inside the complex. I'm grateful, however, that I lugged my camera to India. Not only do I capture some wonderful moments but I also help several pilgrims who want their picture taken at this important spot.

There's no holier place for Hindus to die than Varanasi - the belief is that drawing your final breath here will break the endless cycle of reincarnation. Because of the cremations that unfold at some of the 80-plus ghats - known as burning ghats - it's nicknamed the City of Death.

We see that ritual at a distance from the water, after rising before dawn for a boat ride on the Ganges. Before it ends, we'll each drop a diya - a biodegradable leaf bowl filled with flowers and a burning candle - onto the river's surface as a blessing. After I clumsily stumble on deck, our guide Neha Trivedi Mishra clutches my life vest as I lower my diya onto the water. Better safe than sorry (and I'm not ready to die in Varanasi just yet).

The sun is a slice of orange garnishing the horizon - its colour courtesy of a cocktail of pollution and smoky air. Birds wheel above our heads, the riverside temples and pilgrims glow golden, and I sear the scene into memory. It's hard to know what could top this - but then again, Agra is our next stop. Hello to another basic train bunk bed as we rattle over 550 kilometres of countryside.

Confession time: one of my main motivations for taking this trip is that it gives me a chance to finally admire the Taj Mahal - the 17th-century mausoleum and monument to love that's considered one of the world's most beautiful buildings. Its opulent curves do not disappoint.

The thing that surprises me most about visiting the Taj is that, despite the crowds, it's an utterly peaceful experience. Professional photographers are also busy snapping a continual supply of visitors keen to pose on the bench made famous by Princess Diana in 1992 (Prince William and Kate later sat on the same bench in 2016).

Near Agra is the Wildlife SOS Elephant Conservation and Care Centre - home to dozens of rescued Asian elephants. We learn that the animals were used for tourism, processions, performances, labour and street begging, with private owners all over India considering them status symbols.

"Elephants are exploited in so many different ways," says a staff member. Tourists to India who ride or paint on elephants, or take selfies with one, usually have no idea how brutally the animals have been treated in order to break their spirit and have them cooperate with humans. Among the centre's triumphs is the rescue of Lakshmi - a former "begging" elephant who was so malnourished for so long that she became known as "India's skinniest elephant".

A visit to the centre is a hands-off, observe-from-a-distance, eye-opening experience - but also extremely poignant. In the grounds is an Elephant Memorial with headstones that movingly honour individual animals that have died.

The centre also has a gift shop stuffed with elephant-themed goodies. Even though I've sworn off buying souvenirs, I end up going home with a cute tote and incense holders. That's the charming thing about India - in among all the colour and chaos, it somehow has a way of changing your mind.

The writer was a guest of Explore Worldwide