Officer believes someone involved in teen's death

Mark Haines was 17 when his body was found on train tracks. (Dean Lewins/AAP PHOTOS)

A police officer involved in re-investigating the death of Gomeroi teenager Mark Haines has told an inquest he's always believed someone was involved.

Mark Haines was 17 when his body was found on train tracks just outside Tamworth in northern NSW in January 1988.