'I'll just get you to stand back across the other side of that track for a moment," says Sean Morris as he flips down his safety glasses. "I'm going to be making a bit of noise and a few sparks here."
It is, it turns out, a piece of classically Kiwi understatement - the kind popular with men who make their living doing dangerous stuff, particularly when they're in the presence of somebody whose clean fingernails suggest his most risky daily undertaking is putting boiling water on a tea bag.
By the time I've got my feet, weighed down by safety boots one size too big, at what Morris has deemed a safe distance, he's lowering the high-speed spinning blade of the rail saw on to the mark he's made on the steel and all hell is breaking loose.
I'm glad of the industrial-strength earmuffs he lent me (along with the boots) before he got to work.
Morris and his mate, Dan Bettridge, make one of two gangs - the other is Phil Bernard and Louis Matautia - working through the night on a 600m stretch of the main trunk line near the Parnell Baths.
It's after midnight when they start work and the trains have long since stopped running, but there's a dedicated safety officer, Travis Corbett, with the somewhat sci-fi job title of "Protector". He's in charge of making sure that no train comes hurtling through the work site, a danger more theoretical than actual so close to Britomart, he agrees.
Fittingly, we've started with a detailed safety briefing. As container-laden behemoths of the road turn out of the terminal gate and labour up through the gears towards the motorways, we stand around in the glare of the truck-mounted lights and Morris goes through a checklist of hazards. He laces his warnings with flashes of blokeish humour: when another visitor asks whether there are any hazardous gases, Morris deadpans, "I don't know if Dan's been eating baked beans, but otherwise you should be right."
The order of the night is destressing the rails. At least three of the men have a go at explaining to me what this is - there are mentions of metal's neutral state and expansion rates (0.017mm per metre per deg C above 32), in case you're interested)- but my eyes keep glazing over.
What is plain is that a rail under stress can buckle. "If you get a buckle here, the train will most probably end up in the tide," says Corbett, pointing into the small bay beyond the embankment, "and we don't want to have to give Len Brown a call in the morning."
Continuous welded rail (also known as ribbon rail) replaced jointed rail last century and has been the industry standard since the 1950s. It's dearer to lay but cheaper to maintain. But it keeps the destress crews busy, day in, night out, attending to the 4000km of track in the three zones of the KiwiRail network.
Morris chucks the 20mm slice of steel rail, stovetop-hot from the friction of the cuts, on to a designated "hot spot" between the rails, marked by a fluoro-paint cross. Hydraulic tensors attached to the rails pull the steel like a rubber band, stretching and slackening it repeatedly to close the gap and take the tension out of the line.
The finishing touch is a thermite weld - a chemical reaction between aluminium powder and a metal oxide - that takes place in a crucible set atop a carefully constructed ceramic housing. It's scarcely new technology - the process was patented in the 1890s - but it is spectacular.
With blinding flames and flashes it creates instant molten metal that drops into the mould. The edges are trimmed while it's still glowing, ground while it's still hot and, within minutes, buffed and polished until it's indistinguishable from the steel rail on either side. You'd need expert eyes to see the join today.
Throughout the process, I've been struck by how traditional the work practices are. The most sophisticated technology, apart from a pocket calculator, is hydraulic. Calculations are done according to old printed tables and mathematical formulas; much of the labour is with sledgehammers and spikes, wedges and spacers.
It takes a lot of work to keep a rail network running smoothly, but you get the sense this crew's Victorian-era counterparts would know what to do if they were asked to lend a hand.