The team cleaning between the lines

HIGH WIRE ACT: Linemen have been replacing the 350m of electrical wires above the Makatote viaduct on the North Island main trunk line.PHOTO/ DARRIN CROW 14072015WCSUPVIADUCT

Wrapping up to ward off the cold and rain is a smart move in winter - and it's proving its worth on a multimillion-dollar contract on the North Island main trunk line.

The historic 296 metre Makatote viaduct, about 12km south of National Park, is being sandblasted, strengthened and repainted as part of a $13 million upgrade.

And while the job is being carried out in a harsh winter environment, contractor TBS Farnworth has contingencies to ensure the work carries on.

Graham Matthews, a director of the company, said they were 10 months and half way through the contract. The linemen are on track to finish the job in December next year as scheduled, he said.

He said scaffolding was all the way to the northern side of the viaduct and workers were now on the second to last of eight major trestles supporting the 80m high viaduct over the Makatote stream.

"We'll be wrapping the last section in the coming weeks," Mr Matthews said.

The "wrapping" involves putting a double layer of plastic film around the structure, firstly to contain lead-based paint that is being sandblasted off the steelwork, and then providing the proper environment for the steelwork to be repainted.

While the old paint is sandblasted off the steelwork, the waste has to be collected inside plastic wrapped around the structure and then vacuumed up to a storage site about 100m south of the bridge for later disposal.

The weather is one of a number of challenges with the contract. As well as containing the lead paint, the Makatote stream is home to the rare blue duck (whio) and brown trout, and a condition of the contract is that water quality of the stream is monitored weekly.

But the biggest challenge is keeping rain water out of the work site.

"The double layers of cling film go under the rail line itself but every time a train goes over the viaduct there's a fair amount of movement. This sometimes causes the seals in the plastic to leak. It's the rainwater rather than the cold that's the biggest frustration," Mr Matthews said.

He said doubling the thickness of plastic had created the ideal environment for workers painting the steelwork.

"It can be zero degrees outside but 17 degrees inside thanks to gas heaters we've been placing in the work area. If we can keep the rain water out then we're okay."

He said the decision to work through winter was not a difficult call to make.

"We've got about 60 per cent of the workers from the local area and they've been doing a great job. To stand them down would have been difficult."

Another part of the job has entailed renewing the electric cabling above the viaduct and recently linesmen replaced two 350m long non-tension wires. All work was done between train services