The reward and another coronial inquest could bring answers for his grieving family.
Mark was 17 when he died near Tamworth on January 16, 1988.
A stolen car was found crashed nearby and police concluded the Gomeroi teenager was laid on the tracks either deliberately or in a dazed state. His family disagreed.
Don Craigie, Mark's uncle, has been campaigning for justice for decades, believing there was foul play and police didn't take the investigation seriously.
Uncle Don said Mark had been a happy teenager who had many friends and a girlfriend.
"He was a shy, unassuming lad who was into his football and they used to go to the PCYC," Mr Craigie said.
"Mark was our boy and we knew right from the start that what police were saying was out of character."
After a 1989 coronial inquest returned an open finding, a fresh inquest was announced in 2023 and will begin in April 2024.
At a press conference in Tamworth on Friday, NSW Police announced they were doubling the reward for information from $500,000, which was set on the 30th anniversary of Mr Haines' death in 2018.
Senior police were joined by members of the Haines family to make the announcement, and NSW Greens spokesperson for First Nations' justice, Sue Higginson.
"Uncle Don has never given up on getting to the truth and justice for Mark," Ms Higginson said.
"For over 35 years he has walked a long and rough road to justice and now we have reached a significant bridge in that road.
"What happened to Mark was tragic and the failed police response at the time has compounded the injustice for too long."
Detective Acting Superintendent Jason Darcy said police are committed to finding answers.
"Mark's family deserves closure and peace, having dealt with uncertainty of what happened over three decades ago," he said.
"We believe there are people in the community who have vital information about Mark's disappearance and know exactly what happened to him on that day.
"Any information – no matter how small you may think it is, could be the missing piece to the puzzle."
Uncle Don made an emotional plea for anyone who knows anything about the circumstances surrounding Mark's death to come forward.
"I would love to see somebody claim that million dollars," he said.
"We urge people to put yourselves in our shoes – we just want to know what has happened to our much-loved Mark."
Information should be reported to Crime Stoppers on 1800 333 000 or at nsw.crimestoppers.com.au
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