(Reuters) - When Canadian Pacific (>> Canadian Pacific Railway Limited) CEO Hunter Harrison lays out his case for a takeover of railway peer Norfolk Southern Corp, (>> Norfolk Southern Corp.) he will have unusual company in the form of the railroad's biggest shareholder, activist investor William Ackman.

Canadian Pacific said on Monday that Ackman, who sits on its board of directors and whose $14.8 billion hedge fund firm Pershing Square Capital Management owns a 9.1 percent stake, would take part in a teleconference with the investment community on Tuesday together with Harrison and his top executives.

Four days ago, Norfolk Southern rejected Canadian Pacific's $28 billion acquisition offer, arguing that regulators would reject a deal and that it would cost the company revenue and "harm" the communities it serves.

Ackman handpicked Harrison as CEO after winning a bitter proxy contest at Canadian Pacific in 2012. He has never participated before in any teleconference hosted by the company.

Ackman is no stranger, however, to conference calls or presentations to support an investment thesis. In late October, he spoke for four hours supporting his investment in Valeant Pharmaceuticals International Inc (>> Valeant Pharmaceuticals Intl Inc) after the drugmaker's pricing and accounting practices attracted criticism.

But 2015 is on course to be a terrible year for the billionaire investor, with Pershing Square off 17 percent during the first 11 months after having gained 40 percent in 2014.

(Reporting by Svea Herbst-Bayliss in Boston; editing by Grant McCool)