Fortescue Metals Group (FMG) says it is confident an Australian Competition Tribunal decision to block its access to Rio Tinto rail lines will not impact on operations.

The miner had been seeking access to Rio's Hamersley and Robe railway lines in the Pilbara to transport its iron ore to port.

After nine years of legal proceedings, a High Court decision in September referred the matter to the Tribunal, which today ruled Rio does not have to open its railway lines to other operators.

In a statement, Rio said its operations would have been severely hindered if third parties had been allowed to run trains on the network.

Morning Star resource analyst Matthew Hodge agrees it is unlikely Fortescue will be impacted by the decision.

"They've put infrastructure in place to expand out to 155 million tonnes," he said.

"So they're spending the money to do that. They've got the financing in place and the iron ore price is quite favourable right now so they'll just be planning on and doing what they were going to do anyway.

"I think they lose an option to potentially get some free tonnes down Rio Tinto's railway line, that's pretty much all it means.

"Otherwise they would have had to build their own but that's what they're going to do anyway."

Mr Hodge says if access had been allowed, it may have opened up access to the iron ore industry.

"If you build something, you operate it, you own it, you would think that you have control over that," he said.

"It did look for a while that the lines were going to be declared. There's obviously been a lot of legal action since then to get us to this point.

"If it'd happened it would have lowered the barrier to entry for new competitors in the iron ore place."