He was freed shortly after 9.30 and taken to Waikato Hospital, said a St John Ambulance spokeswoman.
Late last night, his condition was given as serious but stable.
Police Inspector Peter Raynes said the truck-and-trailer unit had "slipped" as it crossed the lines, but the exact cause of the accident was still being investigated.
KiwiRail spokeswoman Jenni Austin said one minor freight line that serviced the Kinleith Mill was affected, but there were no disruptions to any scheduled train services.