Due to its high volume and high value of traffic to NZR and the steep grades in the central section, the NIMT has seen the use of the most powerful locomotives in New Zealand.

When the NIMT opened in 1909, the powerful 4-8-2 X class was introduced to handle heavy traffic over the mountainous central North Island section.

Three G class Garratt-type locomotives were introduced in 1928, but these were not as effective as anticipated.
In 1932 the 4-8-4 K class was introduced, and later improved in 1939 with the Kᴬ class.

The introduction of the DF class in 1954 began the end of the steam era, and in 1955 with the introduction of the Dᴬ class locomotive, major withdrawals of steam locomotives began.
1972 saw the introduction of DX class locomotives and the Silver Fern railcars; the latter remaining in service between Auckland and Wellington until 1991.

With electrification and the introduction of the EF class electric locomotives in the late 1980s, the DX class was mainly reassigned to other areas of the network, including hauling coal on the Midland Line in the South Island.
Since then services between Te Rapa and Palmerston North have been worked mainly by the electrics, although some services are still diesel-operated, such as those originating from or terminating on other lines, or originating from within the central section, like the paper pulp freight trains from Karioi.

As of May 2020, regular rolling stock on the NIMT include:
DC class – all sections
DFT class – all sections
DL class – all sections
DX class – all sections
EF class – Palmerston North to Te Rapa
Due to its high volume and high value of traffic to NZR and the steep grades in the central section, the NIMT has seen the use of the most powerful locomotives in New Zealand.
When the NIMT opened in 1909, the powerful 4-8-2 X class was introduced to handle heavy traffic over the mountainous central North Island section.
Three G class Garratt-type locomotives were introduced in 1928, but these were not as effective as anticipated. In 1932 the 4-8-4 K class was introduced, and later improved in 1939 with the Kᴬ class.
The introduction of the DF class in 1954 began the end of the steam era, and in 1955 with the introduction of the Dᴬ class locomotive, major withdrawals of steam locomotives began. 1972 saw the introduction of DX class locomotives and the Silver Fern railcars; the latter remaining in service between Auckland and Wellington until 1991.
With electrification and the introduction of the EF class electric locomotives in the late 1980s, the DX class was mainly reassigned to other areas of the network, including hauling coal on the Midland Line in the South Island. Since then services between Te Rapa and Palmerston North have been worked mainly by the electrics, although some services are still diesel-operated, such as those originating from or terminating on other lines, or originating from within the central section, like the paper pulp freight trains from Karioi.
As of May 2020, regular rolling stock on the NIMT include:
DC class – all sections
DFT class – all sections
DL class – all sections
DX class – all sections
EF class – Palmerston North to Te Rapa
North Island Main Trunk