KiwiRail has admitted mistakes were made after a group of contractors was affected by gas while working in the country's longest tunnel in August last year.
The Rail and Maritime Transport Union said 10 contractors working in the 8.9km Kaimai Tunnel were almost overcome by gas and could have been killed.
Union spokesperson Wayne Butson said one group did not leave the tunnel straight away when their gas monitor alarm sounded and the men became weak and disorientated before walking out the wrong end of the tunnel.
Mr Butson said they would have inhaled a cocktail of fumes from compressors, generators and other machinery mixed with dust which would have led to nausea, disorientation and giddiness.
He said the accident happened because five of the men had not been trained in tunnel gas awareness.
Lawyer Hazel Armstrong who specialises in health and safety was asked to investigate the incident by the union.
She said the men left the tunnel the wrong way and walked into the gas. "For some three kilometres they were walking or trying to walk through these toxic fumes. They were lucky to get out. By the time they got out they were gasping for breath."
KiwiRail has admitted the men should have had more instruction and said all staff now receive training on tunnel gas.
"We recognise that there was a carbon monoxide issue on the day, we've reviewed those issues, including that, and ensured that our procedures going forward make the tunnel safe for people to work in," spokesperson Rick van Barneveld said.
The union is angry the men were not checked for poisoning after the exposure and were instead taken by a KiwiRail manager for some food and drink to recover.
Mr van Barneveld said the company believes the manager acted appropriately at the time, but it would in future follow through a similar incident more comprehensively.
He said KiwiRail has reviewed its procedures and a focus group will meet in two weeks to review the incident in more detail.