Initial results from the second round of testing for asbestos on KiwiRail's fleet of Chinese-made locomotives have come back negative.
The Rail and Maritime Transport Union received the first five test results on Monday and admits it was surprised by them.
Union secretary Wayne Butson says the results don't align with what members are telling the union.
KiwiRail removed 40 freight locomotives from service at the end of February, after the manufacturer confirmed the highly toxic material had been sprayed on metal sheeting in the engine room.
While testing was underway asbestos was also found in the doors and work has begun to remove this.
Last week, KiwiRail and the union released the first round of test results which showed there was no airborne asbestos; but the union was unhappy with those tests.
A second round of tests, being conducted to union specifications, should be completed this week.
Mr Butson says these latest results mean there were no asbestos fibres found on the five locomotives tested and show the asbestos is fully contained.
But Mr Butson says the tests were carried out on the second generation locomotives, which are up to two years old.
He says he doesn't expect the same results from the first generation locomotives, which are up to four years old, because asbestos is known to break down and degrade over time.
The fleet of 40 locomotives is evenly split between the first and second generation locomotives.
Mr Butson says they will await all the test results before deciding what action to take, but the union isn't ruling out allowing the second generation fleet to return to service if they return negative tests.
KiwiRail declined to be interviewed, but it has said it will not allow the locomotives to return to service until it is absolutely certain they are safe.