KiwiRail is being accused of breaching a collective agreement with the Rail and Maritime Union.
The state owned operator is accused of outsourcing maintenance work on locomotives to Chinese-based company CNR.
Rail and Maritime Union and Kiwirail are going before the Employment Relations Authority over the next three days to sort the dispute.
Union spokesman Todd Valster said they had the staff and the workshops here in New Zealand to do the work. The case would also cover the working conditions of those doing the work on behalf of CNR.
Valster said employees may be getting paid more or less than the minimum wage, but they haven't been able to get a satisfactory answer from KiwiRail.