My fellow train passenger collapsed in front of me. I couldn't stop laughing

I was sitting chatting to an English backpacker sitting opposite me on the train when all his colour suddenly drained away, his eyes became glassy, and he slumped forward, face-first into his bright yellow lunch of saffron chicken and rice.

My reaction surprised me: I laughed. And once I started, I couldn't stop.

We were travelling on the Ferrocarril Central Andino train from the Peruvian capital Lima to Huancayo, tucked into the Andes, on the highest railroad in the Americas, and the second highest in the world. Only the Qingha to Tibet railway in China is loftier.

And it wasn't until then, close to the point that it soars 4783 metres above sea level, that I learnt what a terrible effect high altitude has on me. While the thin air knocks out others, it makes me laugh hysterically, and I just can't stop.

(Despite my laughter, altitude sickness is a serious matter. It pays to take precautions and gradually acclimatise for travel at altitude.)

All around me on that 14-hour, 420-kilometre journey, there were clear signs of alarm. Other passengers rushed up to the fellow traveller, gently lifted him from his plate - and the sight of his features encrusted in food made me, I'm sorry to say, laugh even harder - and shouted for the doctor.

The man who'd been cooking the meals rushed in, swapping his greasy yellow apron for a white coat as he approached our seats. He brought with him a big cushion with a nozzle attached, which he pushed at the backpacker's nose while squeezing the cushion under his arm, rather like bagpipes.

It contained pure oxygen and his patient soon came around. When he eventually sat up, bewildered over what had just happened, I couldn't contain my hysteria.

There are always highs in travel, and then there are real highs, and South America is exceptional for both.

That train trip, for instance, is an astonishing journey, wending its way through 69 tunnels, 58 bridges, and zigzagging over seven switchbacks.

The scenery is incredible, ascending from the parched coastal plains, up the sides of dramatic mountains, across gorges and past deep ochre cliffs. There's the grey mining town of Casapalca along the way, cluttered with industrial equipment to produce zinc and lead, and countless sheer, dizzying drops, misty with clouds.

Everywhere, there are splendid views of snow-capped Andean Mountain peaks until we descend once more towards our destination in the lush Mantaro Valley.

But inside the carriages, with the thin air at altitude making so many sick, it's just as colourful.

Even as the backpacker has come back to life, a woman collapses on the other side of the aisle and another rushes to revive her. And horribly, as she comes to, I start laughing again at the sight.

I'm mortified, but I just can't stop. Then a couple holding hands by an open window are splattered by filthy water from a snapped pipe hanging down over the line from coal slagheaps by the side of the track, and tears of mirth run down my face.

One of the other passengers shakes his head. "It's the thin air," he says. "It affects us all differently."

That journey was one of the highlights (no pun intended) of various visits to South America, but it also provided a salutary lesson on how altitude affects me. After that, whenever I am climbing high, I ready myself mentally for an attack of hysteria and apologise in advance to anyone with me.

That approach served me well in Peru's Cusco (3399 metres high) and giggling along the Inca Trail to Machu Picchu which reaches a height of 4215 metres at Dead Woman's Pass. It also worked nicely for the Ecuadorian capital Quito (2850 metres) and in Colombia's Bogota (2640 metres).

But the laughter finally died when I hit Bolivia's La Paz (3640 metres) where I suffered altitude sickness so severely, I ended up in bed for a week.

There, I felt so terrible, I couldn't raise so much as a smile, not even at the memory of one of the greatest experiences of my travel life along that Lima to Huancayo railway, accompanied by my longest-ever laughter track.