(Reuters) - CSX Corp CSX.O shares skidded as much as 10 percent on Friday, wiping out $4 billion in market value, as the No. 3 U.S. railroad by revenue sought to assure investors its turnaround would progress despite the unexpected medical leave of its chief executive officer.

FILE PHOTO: Hunter Harrison, CEO of Canadian Pacific Railway Limited speaks to the economic community at a business luncheon in Toronto, March 2, 2015. REUTERS/Mark Blinch/File Photo

CSX stock, which has soared nearly 60 percent this year, tumbled 7.3 percent to $53.11 in midafternoon trading after earlier falling as low as $51.63.

Most of the gains came after Hunter Harrison, 73, who led turnarounds of two Canadian railroads, was hired as CEO in March in a push by activist investor Paul Hilal.

Harrison has tried to reform 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 railroad in March, but customer complaints and service disruptions came to light over the summer and continued at least through October, drawing federal scrutiny.

Acting CEO Jim Foote, who worked with Harrison at Canadian National Railway CNR.TO, insisted he will carry on the CEO's plans, but offered no details on his health or when he might return to the helm of the Jacksonville, Florida-based company.

Harrison previously took a medical leave at Canadian Pacific in 2015 after surgery and a bout with pneumonia.

“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 Hilal’s new investment firm Mantle Ridge for arranging Harrison’s appointment and gave the septuagenarian a four-year contract estimated at $300 million.

Hilal, who now sits on the CSX board, was instrumental in appointing Harrison to run Canadian Pacific Ltd CP.TO when Hilal worked at William Ackman's Pershing Square Capital Management. The bet on CP earned the hedge fund $2.5 billion.

Hilal did not return requests for comment.

RAISING QUESTIONS

With Foote now running the show, investors showed some nervousness about whether the executive can deliver on Harrison’s ambitious plans.

“It will be a classic ‘show me’ story,” said Taylor Glasebrook, an associate portfolio manager at Neuberger Berman, which owns 10.5 million shares of CSX, making it the No. 8 shareholder in CSX.

“One investor concern may be that any possible changes to the pace of margin improvements (envisioned by Harrison) could slow down,” he said.

Even before taking on the CEO job, Foote had his hands full as chief operating officer and chief sales and marketing officer. “There could be concerns among investors about whether Jim can wear three hats,” Glasebrook added.

CSX will give investors details on the company’s strategy, such as possible plans to sell off some short-line rail segments, at its investor conference in early March, Foote said.

Given Harrison’s age and health problems - he was seen using an oxygen tank when meeting with investors last month - investors want to know more about the company’s succession plans.

Harrison on Nov. 29 said 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 acquire competitors. He made a failed bid for No. 4 U.S. railroad Norfolk Southern Corp NSC.N, backed by CP shareholder Pershing Square and held abortive talks to buy CSX.

Foote, who previously 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, adding that Harrison held management training sessions and spoke at an analyst conference in recent days.

Foote said CSX has already gone through the “real heavy lifting,” including closing numerous rail yards where train cars are sorted.

But 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.