Things don’t come more Da Vinci Code than a ride on the Pope’s railway – the centrepiece of a new tour launched by the Vatican Museums this month. The train departs the Vatican City’s private railway station – a marble building in the shadow of St Peter’s basilica – on a stretch of track that originates in a tunnel dug into the hillside beneath the Vatican Gardens. It travels along the 300-metre Vatican railway line – the shortest in the world – into Rome, then rumbles on to the Pope’s sumptuous Castel Gandolfo summer palace in the Alban hills south of the city.

This day-long excursion combines existing tours of the Vatican Museums and gardens with a tour of the grounds of the Castel Gandolfo. And for the first time the locations are linked by a ride on the Papal railway. At €40pp, it’s also the cheapest way to explore the papal properties: a guided tour of the museums and gardens in the Vatican alone costs €32.

People visit the gardens of the pope’s summer residence, Castel Gandolfo, south of Rome. Photograph: Vincenzo Pinto/AFP/Getty Images

I joined the tour on its inaugural Saturday, my pre-booked ticket letting me dodge the queue that, at 8am, already snaked around the walls of the Vatican City. The first two hours are unchaperoned, so my recommendation would be to avoid the Sistine Chapel cattle-run and visit the quiet contemporary religious art galleries, which include works by Dali and Matisse.

The guided walk through the Vatican gardens that follows is the perfect antidote to the rammed museums. It meanders between fountains and grottos, past the monastery where the retired pope, Benedict XVI, lives, and on to Stazione Vaticana, the city’s only station.

The train – ordinary Trenitalia rolling stock, sadly, not the Papal steam train – takes an hour to reach the town of Albano Laziale, where a shuttle bus takes passengers to the summer palace (a separate tour of the palace interior is available at extra cost). Tourism in the towns around the palace suffered when Pope Francis decided not to spend his summer there, and this decision to open it to the public has been interpreted as a helping hand.

Aerial shot of the Vatican gardens from St Peter’s Basilica, Rome. Photograph: Alamy

The tour of the grounds, aboard a tiny land train, is revelatory. Formal gardens are interrupted by fields of alfalfa grazed by the cows that provide milk to the Vatican. Roman ruins – including a theatre built for the Emperor Domitian in the first century AD – jut out from behind fragrant trees.

Back in Albano Laziale, some exploration leads me to organic, vegetarian restaurant Le Bioalchimie (Via San Filippo Neri 16) for a €15 buffet lunch of local produce. The train back to Rome departs Albano Laziale at 5.18pm, depositing us at San Pietro, a station just outside the Vatican walls.

A stroll to St Peter’s Square, where high walls obscure the gardens and the Basilica hides the 60-metre Vatican hill, makes for a tantalising end to a fascinating day.

• Vatican by Train tours run on Saturdays only at 8am. Book online at biglietteriamusei.vatican.va/musei/tickets/do