TCI, which owns over 5% of Canadian National, said it hired Kingsdale Advisors, a firm which had represented Bill Ackman in his efforts to bring about changes at Canadian Pacific Railway.
"We believe CN's best days are ahead of it", provided the company pulls out of its $29 billion bid to buy Kansas City Southern, TCI's Hohn said.
The fund expects to nominate at least five new directors, he added.
Canadian National, which has 11 directors on its board, said it would make carefully considered decisions in the interest of its shareholders and in line with the company's strategic priorities.
The fund reiterated its call for CN's top boss Jean-Jacques Ruest to be replaced by former Union Pacific Chief Operating Officer Jim Vena.
TCI, the company's second largest shareholder, as per Refinitiv data has held a stake in since 2018.
(Reporting by Nathan Gomes in Bengaluru; Editing by Shailesh Kuber)