Rail workers are still concerned about possible asbestos on KiwiRail's Chinese-built locomotives despite tests showing only six were affected.
The Rail and Maritime Transport Union said a plan to return the 40 DL locomotives to service was premature.
An initial round of tests were negative for airborne asbestos, but the second round showed presence of asbestos in some locomotives and loose fibres in one of them.
"The presence of loose fibres is of huge concern to us, as it is loose fibres that have the potential to be respirable," union general secretary Wayne Butson said.
"We need to see a comparison between the first and second round of testing for each locomotive, in order to understand why some have gone from negative to positive."
KiwiRail said yesterday that seven out of the 204 samples taken from the locomotives showed a small presence of non-respirable asbestos in some locomotives and fibres in one. It found no asbestos dust in the remaining 34 locomotives.
The 40 locomotives, which are normally used on freight trains, were removed from service last month after it was found the highly toxic material had been included in a compound sprayed on metal sheeting in the engine cabs.
KiwiRail and union officials are meeting in Auckland today and tomorrow to discuss the test results.
KiwiRail chief executive Peter Reidy said the company was confident measures could be put in place to bring the locomotives back into service soon, a statement the union described as "presumptive".
- Fairfax Media