You'd think that the man caught mid-squat beside the track would have known he was risking being photographed by several hundred international tourists. The TranzAlpine train service has, after all, been running twice-daily between Christchurch and Greymouth since the 1990s, with only a brief hiatus after the February earthquake; however, as we all know, the call of nature can be hard to deny.
Nature is what this trip is all about. In less than five hours, passengers are whisked from the broad expanse of the Canterbury Plains, waving with wheat and close-cropped by sheep, through the steep-sided drama of the Waimakariri Gorge and the snow-capped peaks of the Southern Alps, to the lush rainforest of the West Coast.
It's all about contrast - and comfort, when viewed from a soft seat through the window of the carriage, refreshments lined up on the table. But I'm after fresh air and photographs, so when the train stops at Springfield halfway across the plains, I get out to stretch my legs and re-board at the open viewing car.
"It gets good when the train turns right," I was told by a staff member, and yes, when the rails curve round into the gorge, suddenly there's the river, a luminous turquoise snaking between shingle banks golden with gorse and broom. Cliffs rise up and the track crosses viaducts hanging above the water, the highest the red steel Staircase, 73m, leading straight into one of the route's 19 tunnels.
From the viewing car, the sudden transition to choking blackness is shocking, even claustrophobic, and it's a relief to emerge again to wide valleys and mountains streaked with scree below and snow above. We zip past Cass, with its cute little hut, and pull in to halt at Arthur's Pass, where a kea perches on the station sign.
Trampers alight with backpacks to explore the alpine scenery: some to stay in the mountain town for serious hiking and climbing; others simply to take a guided walk along the Bealey Valley, working up an appetite for a picnic with the keas before being driven back to Christchurch along the Great Alpine Highway. I stay with the train for the second half: the coast's yin to balance Canterbury's yang.
Having accidentally re-boarded at Springfield on the wrong side of the diesel generator, I'm careful to get it right this time: next up is the Otira Tunnel, and the viewing car is closed. The commentary highlights the 15 years it took to dig and the 15 minutes to travel through it. There's no mistaking where we are when we burst out of the tunnel: the light is softer, the bare bones of the mountains hidden under a blanket of bush, the peaks swirled in mist. Cattle graze in green fields where flame-shaped kahikatea trees stand and weather-beaten cottages hunker down beside the track.
We follow the Grey River now, living up to its name as it leads us to journey's end, where it meets the sea in a struggle that's claimed many a ship. For day-trippers, there's only an hour to explore Greymouth before the train leaves on the return journey: more than enough, some may say cruelly, but a stroll along the stop-bank and through the town passes the time pleasantly.
Others head north to Punakaiki and Westport, or south to Hokitika and the glaciers; for me it's back to Christchurch, just in time for tea.
IF YOU GO
The return journey on the TranzAlpine, from 8.15am until 6.05pm, costs from $79 an adult.
* Visit Pamela Wade's travel blog.