Otamarakau Sandpit Group Stopover campsite caretaker Colin Campbell was working nearby when the accident happened and said he heard the horn of the train going for "ages and ages", before it came to a stop about 400m from the start of the campsite.
Department of Conservation spokesman Rory Newsam said the woman was from a local community group that assisted DOC.
"We're obviously concerned for her wellbeing, and are working with KiwiRail and members of the community."
A KiwiRail spokesman said the train driver would be offered counselling and time off work.
"We do not know why the person was on the tracks but KiwiRail urges the public to stay well clear of the rail line."