Shares of U.S. rail operator CSX Corp. fell 9 percent in the premarket after the company said CEO and President E. Hunter Harrison has taken medical leave after unexpected complications from a recent illness.

Chief Operating Officer James M. Foote will fill in as as acting CEO for Harrison, who earlier this year left as the head of Canadian Pacific to take the the top job at CSX, the nation's third largest railroad operator.

The company will hold a conference call on Friday morning (Dec. 15) at 7:30 a.m. EST to give updates.