Gomeroi teenager Mark Haines was found dead on the tracks south of Tamworth in NSW on the morning of January 16, 1988, near a crashed stolen Holden Torana.
An autopsy found the 17-year-old died from a traumatic head injury.
The initial police investigation concluded he lay down on the tracks either deliberately or in a dazed state, something his family has never believed.
An inquest that opened in mid-2024 has been examining the circumstances of Mark's death and the original police investigation, after a long campaign by his uncle Don Craigie to revisit the case.
Mark's family have long suspected foul play and believe the police investigation was hindered by racism.
During closing submissions on Thursday, counsel assisting Chris McGorey said there were "substantial" deficiencies in the initial police work.
Those flaws included improper collection of evidence from the tracks and the quick removal of Mark's body and the train.
Police also failed to involve detectives and did not immediately interview Mark's family or investigate any link between the teenager and the crashed car.
"(With) a more robust investigation ... the coroner and Mark's family would be far more likely in a position closer to the truth," Mr McGorey told NSW Coroners Court in Lidcombe on Thursday.
It would be up to Deputy State Coroner Harriet Grahame to determine whether racial bias influenced the police investigation, he said.
Describing the investigation into his nephew's death at the inquest in 2025, Mr Craigie said: "That train would still be there if it was a white boy.
"They would have turned that train engine over."
Mr McGorey said it was open for the coroner to accept Mr Craigie's belief about investigators' bias.
"I want to make it clear that there's no dispute from me that there were clear deficiencies in the police and coronial response in 1988 and 1989," he said.
The circumstances leading up to Mark's death largely remained a mystery, with various scenarios possibly unexplored, the coroner was told.
"These are regrettably and tragically ... the unknowables," Mr McGorey said.
"There is two-and-a-half hours of time in which we do not know Mark's movements or what occurred."
The inquest also heard evidence Mark could not drive and would never steal a car.
That meant it was "highly likely" Mark was not alone when he ended up on the tracks, Mr McGorey said.
"These persons have not been forthcoming with what they know, or their identities have not been (revealed)."
Ms Grahame will deliver her findings at a later date.
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