(Adds background on Harrison, analyst comments. Updates stock price)
Dec 15 (Reuters) - CSX Corp shares slid as much as 10 percent on Friday, wiping out $4 billion in market value, as the No. 3 U.S. railroad sought to assure investors its turnaround would progress despite the unexpected medical leave of its chief executive officer.
CSX stock, which has soared nearly 60 percent this year, tumbled 7.2 percent to $53.20 after falling as low as $51.63.
Most of the gains came after Hunter Harrison, who led turnarounds of two Canadian railroads, was hired as CEO in March in a push by activist investor Paul Hilal of investment fund Mantle Ridge LP.
Harrison has overhauled CSX 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 started the overhaul of the No. 3 railroad by revenue in March, but customer complaints and service disruptions came to light over the summer and continued through at least October, drawing federal scrutiny.
Acting CEO Jim Foote insisted he will carry on Harrison's plans but offered no details on Harrison's health or when, 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.
In June, CSX shareholders approved an $84 million payout to Mantle Ridge for arranging Harrison's appointment and gave Harrison a four-year contract estimated at $300 million.
Mantle Ridge's Hilal pushed CSX's board to hire Harrison. He helped install Harrison at the top of Canadian Pacific Railway Ltd.
Hilal was not immediately available for comment on Friday.
CSX investors want to know if the company has succession plans, given Harrison's age and health problems. He occasionally uses an oxygen tank but has said he was fit enough to turn around another railroad. Harrison was seen using a tank at a hearing recently in Washington.
He said on Nov. 29 he was "trying to stay back a little bit" to give Foote and other managers the space to steer CSX, a move he described as part of his succession plan.
"Hunter Harrison is a unique talent and leader and if he is unable to return to the company it would raise real questions over CSX's long-term margin potential," Citi analyst Christian Wetherbee said in a research note.
While at Canadian Pacific, Harrison sought to buy competitors. He made a failed bid for No. 4 U.S. railroad Norfolk Southern Corp, backed by CP shareholder Bill Ackman of Pershing Square Capital Management and held abortive talks to buy CSX.
Foote, who worked for Harrison when he led a turnaround at Canadian National Railway Co and was the CEO of railroad products supplier Bright Rail Energy Inc, has never headed a major railroad.
Foote brushed off questions about whether the board was slow in warning that Harrison has health problems.
"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 said CSX has already gone through the "real heavy lifting," including closing numerous rail yards where train cars are sorted.
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)