The American activist investor who sparked a turnaround of Canadian Pacific Railway has resigned from the company's board of directors.

The Calgary-based company (CP.TO) says Bill Ackman resigned Tuesday after more than four years on the board.

Pershing Square Capital Management L.P., the hedge fund controlled by Ackman, said last month it was selling its entire 9.8-million shares, worth about $1.9 billion.

Ackman led the effort to install a new board and new management, including attracting former CN Rail boss Hunter Harrison out of retirement to head the railway.

"Over the last four years we have built a better CP and that model remains in place to continue to deliver not just for shareholders, but for customers and employees," Harrison said in a news release.

Named to join the board are banker Jill Denham and William Fatt, former chief financial officer of Canadian Pacific Ltd., the former parent company of Canadian Pacific Hotels and the railway.

In separate news, Pershing Square disclosed in a regulatory filing Tuesday that it has a 9.9-per-cent stake in Chipotle Mexican Grill and that it planned to engage in discussions with the company. The Denver-based company struggled after suffering an E. coli outbreak and norovirus cases last year.