ScotRail Alliance, which involves Abellio ScotRail and Network Rail, has been criticised for the quality of services on the line.
A railways expert and MSPs have concerns about the long-term future of the line that links Inverness to stations in Sutherland and Caithness.
But ScotRail Alliance said it was investing in the line.
Last month David Spaven, who writes books on the histories of railway networks, said there had been a recent "downgrading" in the quality of services.
He told BBC Scotland people were being put off using the trains because they were unreliable.
Mr Spaven said: "Sadly in the last two or three we've found that the service has got more unreliable and people have been voting with their feet.
"There are fewer people using the line now and that's at a time when most rail routes in Scotland are seeing mushrooming growth in patronage, but the north line is seeing decline and I think that is a big worry.
"The trains that are using this line are Class 158s which are the most unreliable diesel units in Scotland.
"To be frank, when they came in 25 years ago they were poor trains then and even poorer trains now. They keep breaking down, they've problems climbing hills or the air conditioning doesn't work."
Mr Spaven also said the infrastructure of the Far North Line needed improvements, such as extra double track to help improve reliability of services."
Dr Paul Monaghan, SNP MP for Caithness, Sutherland and Ross and an honorary president of the Friends of the Far North Line, and Highlands and Islands Labour MSP Rhoda Grant have raised similar concerns.
But ScotRail Alliance said the rolling stock was being revamped, a modern digital signalling system was being introduced and miles of track being replaced at a total cost of millions of pounds.
Inverness depot fleet manager, Derek Glasgow, told BBC Scotland the operator was looking at increasing line speeds on level crossings and easing travel around stations at Brora and Georgemas.
He said: "All these things actions are happening now and for the next time table.
"We are improving the signalling, we are changing from an analogue system to a digital system and have also got trains being refurbished. They've had new doors, new seats, new lighting."
The operator of Scotland's railway network has reiterated its commitment to the Far North Line in the Highlands.