There has been a chemical leak on a rail wagon on the North Island main trunk line at Utiku, near Taihape.
Two leaking substances were found in a wagon of dangerous goods on a freight train travelling from Auckland to Wellington, at just after 10am on Tuesday.
The Fire Service says one of the substances has been identified as a construction adhesive, the other is a so-far unidentified corrosive.
Decontamination equipment is being used at the scene. Police say roads near the railway line remain open.
Two KiwiRail employees were taken to Wanganui Hospital for observation.
Passengers on the Overlander train are being carried by bus between Hunterville and Ohakune.