The Rail and Maritime Transport Union says asbestos on some of KiwiRail's Chinese imported locomotives may have degraded to the point where they are potentially lethal to workers.
KiwiRail pulled all 40 of the trains out of service on 27 February after it discovered the highly toxic material had been sprayed on the metal sheeting of the driver's compartment as a form of sound proofing.
The state-owned company has yet to release the results of testing it has been carrying out but union secretary Wayne Butson said initial results showed members could have been exposed.
Mr Butson said one of the inspectors told him on Monday the resin sealing in the asbestos was being corroded by heat, and up to four of the locomotives were in a parlous state.
He said he was told at least two engines should be parked up permanently to prevent workers coming into any further contact with the degraded asbestos.
New Zealand should ban the importing of products that contain asbestos in the same way Australia has, he said.
KiwiRail carried out tests on the new locomotives in 2013 when it was revealed Chinese made trains imported into Australia had tested positive for asbestos.
The company contracted to carry out the checks failed to find any sign of the potentially lethal material and gave KiwiRail the all-clear to continue using the locomotives.
Mr Butson said this has resulted in his members potentially being exposed to the asbestos while driving the trains or carrying out repairs over a period of two years.
He told Radio New Zealand's Morning Report programme workers had been raising questions about asbestos long before it was found last week.
"These locomotives were brought under suspicion shortly after they landed on shore and we've had members on a number of occasions say they think there is asbestos in these locos, and KiwiRail has always denied it."
An expert on the health effects of asbestos said the material is the leading cause of workplace-related deaths in New Zealand with up to 100 fatalities from of mesothelioma cancer every year.
Dave MacLean of Massey University's Centre for Public Health Research said more asbestos is used worldwide than 20 or 30 years ago, and more is produced in developing countries than ever before.
Final tests on the locomotives are due back on Tuesday.