Several fires that merged into one blaze closed the main trunk line near Waiouru yesterday.
Fire and Emergency NZ (Fenz) was alerted to the fires near State Highway 1 at 12.50pm yesterday.
Fenz shift manager Jules Wevley said a Waiouru and Taihape crew were called to four smaller fires near the railway tracks but they merged into one of about 400 by 20 metres so more crews were called.
A tanker, a helicopter and four rural fire crews helped extinguish the fire.
Wevley said the fire was 80 per cent contained by around 3.30 pm, with crews focusing on extinguishing hotspots.