According to the report, Harrison is only seen in the firm's Jacksonville, Fla., headquarters no more than a few days a week, and sometimes equipped with an over-the-shoulder oxygen system.
The concerns come ahead of a crucial shareholder vote on June 5 that will decide whether to reimburse Harrison for compensation he surrendered when he resigned from another railroad to pursue the top job at CSX.
In an interview with the Journal, Harrison said he had been cleared by his doctors to work and that his fellow CSX board members are aware of his medical condition, which sometimes requires him to use the oxygen system.
Harrison "has been and continues to be actively and deeply involved on a daily basis," CSX spokesman Rob Doolittle said. "In the absence of performance questions, as a matter of policy, we do not comment on health-related matters of any CSX executive."