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(Reuters) - CSX Corp's CSX.O shares tumbled on Friday in premarket trading after the No. 3 U.S. railroad said Chief Executive Hunter Harrison was taking medical leave for unexpected complications from a recent unspecified illness.

The leave comes amid the company’s controversial turnaround plan that has drawn customer criticism and scrutiny from regulators. [nL1N1OF03H]

“The importance of Harrison’s influence is immense,” Baird Equity Research Benjamin Hartford wrote in a note. Harrison’s health has been an ongoing concern among investors since his hiring, Hartford added.

A railroad legend, Harrison was hired as CSX's CEO earlier this year. He previously turned around Canadian National and Canadian Pacific Railway Ltd CP.TO.

James Foote, who was named CSX’s chief operating officer in late October, was appointed acting CEO.

CSX’s shares fell 10.3 percent to $51.42 in premarket trade. Up to Thursday’s close, they had risen 15.8 percent since March when Harrison took the helm.