Climbing and descending 6,000ft in 40km, the Bernina Express takes Anthony Lambert past palm trees, Alpine woods, waterfalls and glaciers

Glitzy St Moritz is the starting point of three great railway journeys, but only one offers the sight of glaciers and palm trees in a little over two hours. The Bernina Express achieves this contrast by surmounting the highest rail crossing of the Alps, through the pass that gives the train its name, before a dizzying descent into Val Poschiavo and across the border with Italy to Tirano, 61km from St Moritz. From spring to late summer many trains have open-topped carriages to allow panoramic views over the stupendous scenery - and open up a new relationship with tunnels.

One of the remarkable features of this railway is the vertical distance of 5,994ft between its summit and lowest point and a horizontal distance of only 38.4km between them. Normally a rack mechanism would be required to help trains with such hill-climbing but the Bernina line relies on adhesion with gradients as steep as 1 in 14.

Once through the tunnel outside St Moritz, the train enters the curious area where two river valleys meet the dominant Inn Valley in the Upper Engadine. It is one of the few expanses of flattish ground and is overlooked by the tiny whitewashed chapel of San Gion, picturesquely placed on a knoll with a solitary tree for company. The hillside resort of Pontresina is surrounded by footpath-threaded woods, at least one leading to the Morteratsch Glacier.

When the railway opened this far in 1908, the glacier was within sight of the station; today it's a worthwhile half-hour walk. Starting its climb, the train twists through the first of many hairpin bends so severe that passengers at the rear can see the front going in the opposite direction. A memorable diversion from Bernina Diavolezza station is the cablecar journey to the mountain hotel and restaurant at Diavolezza (9,754ft) for a "view of surpassing grandeur" as Baedeker quaintly put it. Still better is to stay the night and rise before dawn to watch the sun break over the saw-toothed peaks and turn the snow a flamingo pink.

The tree line has been left behind by the time the train skirts the pale green waters of Lago Bianco. Well-shod walkers usually join the train at the summit station of Ospizio Bernina at 7,329ft. Only coarse grasses and stunted bushes grow on this desolate plateau, so remote that a twice daily caravan of 13 mules and three horses had to bring supplies up from Poschiavo to the temporary huts where Italian workers building the railway lived.

The line descends through shelters protecting the sections most prone to avalanches or snow drifts and reaches the station at Alp Grüm. It's again worth breaking the journey, not only to have lunch on the station restaurant terrace overlooking the Palü Glacier, but also to admire the view of Val Poschiavo. The town of Poschiavo is a small bundle of grey shapes 7.5km away but it takes 17.5km of tortuous railway to overcome the vertical drop of 4,035ft.

A series of semicircular bends begins when the train leaves Alp Grüm, dropping down through coniferous woods and tunnels with glimpses through the trees of waterfalls and the valley below. The train enters the darkness and twists round inside the mountain to emerge in a different direction and glide over a chasm of fuming water. The occasional chalet adds a whiff of wood smoke to the smell of pine.

Eventually the train emerges from the woods into upland meadows and arrives at Poschiavo, the principal town of the Italian enclave of Val Poschiavo. Continuing south the train becomes a tram, taking to the street to reach Le Prese with its eponymous Victorian hotel straight out of an Anita Brookner novel.

Jagged walls of rock rise out of the water on the far side of Lago di Poschiavo, rising up to a series of peaks that line the eastern side of the valley. The train curves round the end of the lake and descends steeply to Brusio where there is a unique railway feature: a steeply angled, nine-arched spiral viaduct which allows the line to pass underneath one of the arches before continuing down the valley. Modern sculptures have been placed in the grass within the spiral.

Orchards and market gardens flank the railway as it threads along the street, past front gardens and a tall campanile and across a square on the outskirts of Tirano, where the Bernina Express terminates at a station alongside the Italian Railways' line from Milan.

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The Italian enclave

It's one of those places that induce a feeling of well-being: taking lunch in the main square at Poschiavo with the trickle of water in fountains and splashes of colour from hanging baskets.

A good insight into the valley can be gained from the museum in the Palazzo Mengotti. Recreated kitchen and living rooms are complemented by textile looms which manufacture cloth and linen products for sale. On the southern edge of the town is a row of delightful three-storey villas built around 1830; they were bought by emigrant workers who had done very well for themselves in Spain - the area is still known as the Spanish quarter.

From Poschiavo there is a glorious walk along the Via Alpina through woodland and mountain tracks to San Romerio. You'll see wild raspberries, cranesbill and gunnera, and legions of butterflies. San Romerio is a pretty cluster of stone buildings beside a chapel. The one family that lives there serve hearty lunches of polenta and stew. Do try the aniseed bread that is a feature of the valley.

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