UPDATE 2-CSX shares derail after CEO takes medical leave

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Dec 15 (Reuters) - CSX Corp shares slid on Friday as the No. 3 U.S. railroad sought to assure investors its turnaround would progress full steam ahead despite the unexpected medical leave of chief executive officer Hunter Harrison.

CSX stock, which has soared nearly 60 percent this year, tumbled nearly 8 percent to $52.76. Much of the gains came after industry veteran Harrison was hired to lead CSX in March in a push by activist investor Paul Hilal of investment fund Mantle Ridge LP.

Harrison has been the driving force behind CSX's radical overhaul to boost profits and streamline operations, although his rapid-fire system changes, cost cuts and layoffs triggered service disruptions and drew the ire of customers and regulators.

Harrison, who led turnarounds of several Canadian railroads, started an overhaul in March, but customer complaints and service disruptions came to light over the summer, and continued through at least October, triggering federal scrutiny.

Acting CEO Jim Foote sought to assure investors that he will be able to carry on Harrison's plans but offered no details on Harrison's health or when, if ever, the 73-year-old might return to the helm of the Jacksonville, Florida-based company.

"My thoughts are totally consistent with everything that Hunter and CSX have said to date about what we intend to do. I see no significant material change from those plans," Foote told Reuters by phone after a conference call with analysts.

Mantle Ridge's Hilal pushed CSX's board to hire Harrison with a four-year contract. Hilal helped to install Harrison at the top of Canadian Pacific Railway Ltd. Despite praise from CSX investors, there were concerns about his health. Hilal was not immediately available for comment on Friday.

Harrison occasionally uses an oxygen tank but has insisted that he was fit enough to turn around another railroad. He was seen using the tank at a hearing recently in Washington.

Foote brushed off questions about whether the board was slow in warning of Harrison's deteriorating health.

"The board acted properly and promptly based on the information that we had," Foote said. "As I said, it was a short period of time ago when he was not only actively involved publicly with the financial community but actively involved holding these mentoring meetings with employees, which was really just only days ago."

Foote declined to say whether he would be in contact with Harrison regarding day-to-day operations.

"We just can't talk about it, simple as that," he said. (Reporting by Eric M. Johnson in Seattle; Editing by Chizu Nomiyama, Lisa Von Ahn and Jeffrey Benkoe)