Strathspey Steam Railway needs a crossing on the River Dulnain so it can offer journeys between Aviemore and Grantown-on-Spey.
The group runs locomotives along 10 miles (16km) of line between Aviemore and Broomhill.
It needs three more miles of track (4.8km) to link up with Grantown.

Lewis, 19, signed a three-year contract with the English Premier League club on Friday, having played three FA Cup games for them last season.
Manager Ray McKinnon has also given a contract to Deniz Mehmet following his departure from Port Vale.
Hibernian midfielder Sam Stanton has also made the move to United on a one-year loan deal.
The arrivals come with Cammy Bell, last season's first-choice goalkeeper, possibly leaving Tannadice to return to Kilmarnock.
Bell, 30, is believed to be keen on a move to the Scottish Premiership club.
McKinnon said of Lewis: "I expect him to challenge for the starting position.
"Harry comes here as one of the top young goalkeeping prospects in English football.
"It says so much about his ambition and desire that he has chosen to come up to Scotland to get first-team involvement.
"It is a coup for us to bring him to Tannadice and I am positive we will hear much more about him during his career."
Lewis, who has represented England at youth level, joined Southampton from hometown club Shrewsbury Town in 2015, hopes to challenge for starts with the Scottish Championship club.
"I am excited at the prospect of being involved in first-team football," he told United's website.
"I believe I am joining a winning team that will be challenging at the top of the league and want to play my part in helping the club secure the title."
Stanton becomes United's ninth summer signing and their fourth of the week.
The 24-year-old Mehmet returns to Scottish football after having played with Falkirk before joining Port Vale in January.
Dutch midfielder Jordie Briels also signed on a one-year contract after the 25-year-old left Fortuna Sittard in his homeland.

Iwan Wyn Lewis of Penygroes, Gwynedd, had been sectioned at Ysbyty Gwynedd after allegedly assaulting his mother.
The 36-year-old was visited by an officer in April 2016 about his bail arrangements, which a doctor said could have added to his distress.
His body was found in the Menai Strait on 2 May.
Mr Lewis was receiving treatment at the hospital's Hergest unit and was later moved to the Cynan ward where he received the visit from the officer.
Speaking at the inquest in Caernarfon on Thursday, consultant psychiatrist Dr Olufemi Adebajo said he was "extremely unhappy" about the visit.
"I don't think the police should be able to come to the ward without special permission," he said.
Asked by coroner Nicola Jones whether the conversation with the officer could have added to Mr Lewis' agitation and distress, Dr Adebajo said: "It's quite possible - even likely".
He said he had known Mr Lewis for more than two years and never considered him to be a suicide risk.
The day before his death, he had asked staff for extra medication because he said he was feeling agitated, but they did not change his dosage because they could see no sign of agitation.
On the evening of 2 May, Mr Lewis told staff he was going to the shop, but 45 minutes later his body was found in the Menai Strait.
The inquest continues.
Railway enthusiasts have hired a powerful crane to lift the lower sections of a new bridge into place across a river in the Highlands.
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Dundee United have signed their second goalkeeper in a week by bringing in Harry Lewis on loan from Southampton.
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North Wales Police has been criticised at an inquest for sending an officer to speak to a hospital patient with paranoid schizophrenia.