Bill Achman sells his stakes in Canadian Pacific Railway (CP)

William A. Ackman, the billionaire investor, is selling off a big equity stake in the railway company Canadian Pacific.

Mr. Ackman's Pershing Square Capital Management on Wednesday sold its entire stake of 9.8 million shares in Canadian Pacific Railway in a series of trades arranged by three Wall Street banks. The shares were valued at about $1.5 billion based on Tuesday's closing price. Canadian Pacific shares closed on Wednesday at $147.28.

The move comes during another rough year for the well-known activist investor and his $11.9 billion firm.

As of the end of July, Pershing Square Holdings was down 19 percent for the year. Last year, it lost 20.5 percent.

Mr. Ackman runs a highly concentrated portfolio that places big bets on a small number of companies. As such, Pershing Square is particularly vulnerable to significant declines in the prices of just a few stocks.

Earlier this year, Pershing reduced its stake in Mondelez International, selling 20 million shares in the food and beverage company. It also sold more than 16 million shares in Zoetis, the former animal health arm of Pfizer.

In April, the firm sold 4.1 million of the shares it had in Canadian Pacific, which at the time reduced its overall stake by 30 percent.