Access the Ohakune Old Coach Road from Ohakune or Horopito. It's a one way walk or ride so you'll need to return the same way or arrange transport back.
Start at the the historic Ohakune railway station, ride down Old Coach Road and turn right into Marshalls Road to join the track. Or, drive to the Marshalls Road car park (5 minutes drive from Ohakune) and begin the ride there.
From Horopito (north of Ohakune on SH 4):
Turn onto Matapuna Road from SH4. There is a parking area just across the railway line.