JACKSONVILLE, Fla. (AP) — Shares of CSX are tumbling in premarket trading Friday following the railroad company's announcement that its CEO will take a medical leave. CSX Corp. says Hunter Harrison is on leave "due to unexpected complications from a recent illness." Operating Chief James Foote will serve as acting CEO of the Jacksonville, Florida-based company. Harrison, who's led turnarounds at three other railroads, was hired in March after the Mantle Ridge hedge fund pressured CSX to make changes.