It said miners were the single largest customer group of Canada's railways and accounted for more than half the total rail freight revenue generated each year, yet faced "an unlevel playing field" that left many mining companies "beholden to railway market power".

MAC said the latest changes to the planned Bill C-49 gave the Canada Transportation Agency own-motion power and reduced data transparency relating to final offer arbitration.

"The effectiveness and reliability of rail freight service is critical to Canada's mineral investment competitiveness, and this move by the government will diminish that competitiveness," president and CEO Pierre Gratton said.

MAC counts mining majors Barrick Gold, Rio Tinto Canada and First Quantum Minerals among its members.

Senators had changed the bill a month ago to give miners more power in disputes with railways, Bloomberg reported, however transport minister Marc Garneau would not accept all the changes and planned amendments of his own.

Garneau has tweeted statements praising his support for key amendments to the bill from agricultural groups.

The latest version of the bill must be voted on in the House of Commons then go to the Senate, which could mean further changes, Bloomberg noted.