Bill Ackman’s Pershing Square hedge fund increased its stake in Canadian Pacific Railway during the third quarter, according to a securities filing released Monday . The fund owned more than 15 million shares at the end of September, valued at roughly $1 billion. Pershing Square held just under three million shares at the end June. Ackman previously served as a board member at Canadian Pacific, pushing for a takeover of Norfolk Southern that never happened. He told CNBC in March that selling out of the Canadian Pacific stake was ” one of our greatest investment regrets ” and that he planned to continue adding to the position, in part through exercising forward contracts that he had already purchased. It is not clear how many, if any, additional shares Ackman bought outside of those forward contracts. Ackman is an activist investor, known for pushing for change in a small group of portfolio companies. Elsewhere in the third quarter, he exited his position in Domino’s Pizza and added slightly to his stake in Howard Hughes . Here are Pershing’s equity holdings as of Sept. 30.