In a rainforest wilderness in southern Thailand, Bau and Boi were being attacked by predators. Dozens of them. As soon as the pair stopped on the rainforest trail another would go after their blood. As one sunk its teeth into Bau’s thigh, their guide swiftly dabbed it with mashed-up cigarette. It released its grip, withered and fell to the ground.

Related article: Southern Thailand's hidden gems

“Leeches hate tobacco,” Boi said, grimacing and wiping away the blood. “But I love leeches -- they’re a sign that the forest is healthy. There are so many here because Khao Luang is still full of [animals, like] sambar deer, tapir, binturong and tiger.”

Thankfully there are leeches only in the wetter months. But gibbons whistle and whoop in the trees all year round; forest elephants leave deep round footprints on the jungle paths; clouded leopards stalk the trails at night; and on the high slopes, where hornbills nest and bear cats loll in the trees, there are dozens of endemic orchids. According to local wildlife guide Jens Kühne of Mahachai Tours, Khao Luang National Park is “one of the natural treasure troves of South East Asia”. Yet despite being a short journey from tourist-packed Ko Samui and Phuket, Khao Luang receives almost no visitors. As jets and backpacker-packed buses head for southern Thailand’s busy resorts, daily trains glide serenely out of Bangkok on the Southern Line, passing through a string of ancient royal retreats, Buddhist cities, deserted seashores and tropical forests, including Khao Luang. None of them are well-known, and tourists are considered a curiosity.

“This is the Thailand the royal family has been enjoying for centuries -- a land of glorious temples and palaces, fabulous festivals and forests, and empty beaches and islands,” said Thai tour operator Dee Edwards of Tell Tale Travel.

After the pilgrim town of Nakhon Pathom (which has the largest Buddhist shrine in Thailand), the first major stop on the Southern Line is 78 miles south of Bangkok, Phetchaburi has been a royal retreat since the country was ruled by Rama IV (a history portrayed in the film The King and I). The airy Phra Nakhon Khiri summer palace, where Anna and King Rama often debated, sits high on a rainforest-shrouded hill, overlooking Phetchaburi’s towering chedis (Thai-style Buddhist stupas). The palace is preserved almost as the king left it – complete with 19^th-century furniture and fittings, including a huge, glass-domed observatory. The hills rippling through Phetchaburi’s environs are pocked with ancient Buddha caves, and the long white beaches of Hua Hin, an upmarket spa resort town, are only a few miles away by train. The current king Bhumibol Adulyadej built his own summer retreat in Hua Hin, which is also a good jumping off point for a string of unspoilt tiny coral islands, like Koh Talu.

A few hours further down the line (some 370 miles from Bangkok) is Nakhon Si Thammarat -- a magnificent traditional Buddhist city that lies at the feet of Khao Luang’s rainforest-covered mountains. Nakhon is the Chiang Mai of southern Thailand,– rich with history and culture and packed with temples, including Wat Phra Mahathat Woramahawihaan (or Wat Mahathat, as the locals say), the oldest and most important religious building in southern Thailand, with a towering chedi that is said to contain one of Buddha’s teeth. Nakhon is the best place in Southeast Asia to see shadow-puppet theatre and religious dance, is celebrated throughout Thailand for its unique nielloware-crafted silver and lacquer work, and has been a Buddhist centre since the 2nd Century.

In September, Nakhon hosts one of Thailand’s most vibrant and colourful festivals -- Sat Duan Sip, the festival of the tenth lunar month. Though Buddhist and not Catholic, it is similar to Mexico’s Day of the Dead, when the dividing line between the spirit world and our own is said to be wafer thin, and the karma of deceased loved ones can be altered by performing acts of special merit. Thousands of Thais offer marigolds to the galleries of sitting Buddhas and garland the 250ft-high chedi at Wat Mahathat. Ghostly figures, satin-clad dancers and doll-like beauty queens process through the city’s streets to the pounding of ritual drums. Pageants and shadow-puppet plays are staged under the crumbling old city walls, and parties extend long into the night.

Trang city lies at the end of the Southern Line (432 miles from Bangkok). It is the gateway to a beautiful coast lined with thick tropical forest, rubber tree plantations and little fishing villages, and it is fringed for its entire length with deserted beaches. Waves break against towering domes, and pinnacles of limestone stretch into the turquoise ocean forming archipelagos of jewel-like islands. Ko Muk is a crown of limestone that encircles a sheltered beach accessible only through a sea cave. Ko Kradan is a tiny island washed by gentle coral-filled sea where there is little to do but laze in a hammock, eat sumptuous Thai food and snorkel over the offshore reef. And there are only ever a handful of visiting tourists.

Southern Thailand’s hidden gems