Sometimes you have to get out. Leave. Exit. Dulled by routine, the spirit yearns for a quick escape. The four walls of the overfamiliar local pub – in my case the Dog House in Kennington – begin to close in. Nothing changes, when so much needs to change. It’s time for a decisive departure, a bold break-out, uncomplicated by check-in queues, baggage checks and the tawdry pleasures of airport shopping. When does the next night-train leave?

Climbing aboard the Caledonian Sleeper at around 9pm one evening, I felt an old excitement stirring. The first night-train I took was as a sixth-former, on a school trip to the Soviet Union. A hazy composite memory remains of a crystal-white journey from Moscow to Leningrad: iced breath on a midnight platform; smuggled bottles of wine and a young Russian woman who loved David Bowie and was big in the Young Communist League. We talked Marxism as the USSR slept.

This time my itinerary, hastily drawn up on what should have been the back of a fag-packet, took me from Euston to Fort William and then on a different train to the beautiful coastal town of Mallaig. My ultimate destination was a bar located nearly 600 miles from the Dog House. From Mallaig it’s 45 minutes by boat to Knoydart, a peninsula virtually deserted since the Highland clearances and spectacularly situated between two lochs. There I would stay a couple of nights at the Old Forge, the most remote pub on the British mainland, as certified by the Guinness Book of Records. A pint in the shadow of the glens awaited me.

On the sleeper, there were four bunks in my standard cabin, but only one fellow passenger – John from Southwark, who was going to visit friends who had bought a place on Skye. “It was intended for holidays,” he tells me, “but gradually they found they were spending more time there than in Manchester, the city where they lived. They sort of moved out of urban life. I wish I could do the same sometimes.”

The lounge car was crowded (get there early if you’re hoping for a seat) and as the sleeper meandered through north London suburbs, it was buzzing. A Virgin Pendolino bound for Manchester zipped by. We didn’t care. Night-train passengers, the connoisseurs of rail travel, are in it for the long haul. Over a beer and a plate of haggis and neeps I talked to Geoff, a ponytailed manager at Redhill YMCA.

Every year Geoff buys an All Line Rover pass which offers him unlimited travel around Britain. For two weeks he goes up and down the country and uses the sleeper as an occasional hotel. He is aware this is an unusual holiday choice. “My friends think I’m bonkers, but I’ve done it for four years now.” Outside, Doncaster slumbered as we passed through unnoticed.

Looking out from my cabin the next morning, the first living creature I saw was a deer, observing our progress through the west Highlands with a wary eye. Lonely houses dotted the glens as we passed through Bridge of Orchy and the wild barren expanse of Rannoch Moor. London was a faded metropolitan memory from the night before.

For the next three hours, the train took its time as it journeyed through mist-shrouded hills, skirted deep lochs and paused at tiny Highland stations. It was a beautiful breakfast.

Having arrived at Fort William just after 10am, I was knocking on the door of the Old Forge by early afternoon. No roads lead here. Just an 18-mile hike or the boat across Loch Nevis. The bar was taken over a few years ago by Jean-Pierre Robinet, “JP”, a former sales manager who now spends his time hosting a mixture of walkers determined to conquer a local munro, sailors making their way from Scandinavia, and a local population of a couple of hundred. I fell into none of these categories, having simply taken the scenic route to ordering a pint of Guinness. But the sheer remoteness of Knoydart breeds solidarity among all who visit.

That evening, the harbour began to fill up with travellers pitching up for the night. The Old Forge heaved with diners and drinkers. A group of sailors, on their way to Belfast, described their journey from Bergen to Skye. “Crap weather. We’re calling this holiday ‘Fifty Shades of Grey’.”

Scott, David and Alistair from Aberdeen had walked for nearly 30 miles. “It was virtually all bog,” said David, who seemed somewhat scarred by the experience. The joy of arrival inspires a drinking session that culminates in Alistair learning to tango with a blonde woman from Manchester at 2.30am. According to JP: “The sense of isolation does mean that there are some mad nights here.”

To clear my head the next morning I went for a run in the glen, high above Loch Nevis. Sun and shadow patterned the hills. Suddenly I found the path blocked by a splendid stag. For a few seconds we sized each other up. Then, with stately elegance, he wandered off into the foliage.

London to Knoydart was a journey taken on a whim. But for that encounter alone, it was worth taking the night-train to the far north.

Essentials

Off-peak return from London Euston to Mallaig costs from £165.70 (thetrainline.com). The outward journey takes 13 hours 44 minutes. The Old Forge offers a B&B package of three nights in the Knoydart Snug from Friday 5pm to Monday 10am, including a three-course dinner for two and breakfast, for £300 per person