Authorities have marked their second week of cleaning up a toxic train spill in Queensland.

A train carrying over 800,000 litres of sulphuric acid, spilled when all 26 of its wagons overturned near Julia Creek on December 27, closing a key Mount Isa rail line.

It is unclear just how much of the dangerous substance spilled, with authorities suggesting it may be as little as two carriages’ worth, or about 32,000 litres.

Freight services immediately began to back up, even forcing the popular Inlander service to be replaced by buses.

Wet conditions around New Year hampered the recovery, but reports now say construction on an 800-metre bypass rail line began late last week.

Queensland Rail's Michael Mitchell said dozens of workers were operating around the clock to re-open the line as soon as possible.

“Conditions permitting, we expect construction and certification of the rail deviation to be complete middle of next week,” he said.

The train was on its way to Incitec Pivot’s Phosphate Hill fertiliser factory, and its derailment is estimated to cost $14 million to the company’s net profit after tax, though fertiliser output should be unchanged.

The environmental impacts could soon become greater than the logistical issues.

Water tests have found evidence of sulphuric acid at Horse Creek, a small slow-flowing waterway nearby, which caused authorities to move to neutralise the acid.

Queensland Rail says it is bringing in consultants to undertake environmental monitoring and provide expert advice about environmental impacts.