Lobby group Into the West has warned that if the rail line upgrade between Londonderry and Coleraine is not carried out immediately it could jeopardise the entire future of the line.
Users face having fewer trains travelling at slower speeds, or boarding coaches between rail stations in the city and Coleraine, from January 2012.
Into the West spokesman Jim McBride said: "Investment in the Derry line is long overdue. It has been starved of investment for decades."
He expressed concern that briefing papers given to Regional Development Minister Danny Kennedy point out that a key feature of the plan to deal with financial problems will mean decoupling the Derry to Coleraine link from the network.
The documents go on to say that such a move is necessary because a delay in the £75m upgrade makes it unsafe for trains to operate at full speed.
"The term 'decoupling' is very concerning," Mr McBride said. "This singles it out from the rest of the network. Why are they cutting back rail services which are growing in popularity?
"It takes over an hour on the bus between Derry and Coleraine and the bus substitute will take even longer as it stops in Bellarena and Castlerock. That is not a viable alternative to a 40-45 minute rail journey.
"Our fear is that the bus substitution is only the first step towards eventual closure.
"We were promised an improved train service, we were promised track upgrade and we were promised some of the additional new trains to the Derry line - and we are not getting any of them."