(Reuters) - CSX Corp CSX.O said on Thursday its Chief Executive Officer Hunter Harrison was taking medical leave, an announcement that comes amid of a controversial turnaround plan at the No. 3 U.S. railroad that has drawn customer criticism and scrutiny from regulators. The Jacksonville, Florida-based railroad said Harrison, 73, was taking a leave of absence due to unexpected complications from a recent unspecified illness.

FILE PHOTO - Hunter Harrison speaks to the economic community at a business luncheon in Toronto, March 2, 2015. REUTERS/Mark Blinch

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

"Hunter is a good friend and has been a colleague of mine for many years," Foote, who previously worked under Harrison at Canadian National Railway Co CNR.TO, said in a statement. "He is an icon in the industry and we pray for his speedy recovery."

A railroading legend, Harrison was installed as CEO of CSX earlier this year. He has previously turned around Canadian National and Canadian Pacific Railway Ltd CP.TO

Investors have had high hopes that Harrison would boost profits at CSX and despite customer complaints and persistent service issues at the railroad through the summer months, the company’s stock is up just shy of 60 percent year to date.

One of the concerns raised ahead of the septuagenarian’s appointment to head CSX was focused on his health. Harrison occasionally uses an oxygen tank but has insisted that he was fit enough to turn around another railroad.

Harrison’s methods since joining the company left customers complaining of delays and congestion on the railroad’s network.

At a hearing held by top U.S. rail regulator the Surface Transportation Board in October, customers critical of CSX’s overhaul demanded greater accountability and fewer delays.

Just last week, Reuters reported that the U.S. Government Accountability Office is launching a probe into the safety of increasingly long freight trains being operated by CSX Corp CSX.O to boost profitability.

In after-market trading, CSX shares were unchanged at $57.31.