In 1980 lawyers filing merger paperwork with the Interstate Commerce Commission combined a C, an S and an X as a temporary name for the union of the Chessie System and the Family Lines, and it was never changed. The roots run to the Baltimore and Ohio in the 1820s. The Chesapeake and Ohio absorbed the B&O and Western Maryland to form the Chessie System, known for its sleeping cat mascot, while the Seaboard Air Line and Atlantic Coast Line formed the Family Lines. CSX did not operate as a single railroad until 1987. In 1996 it bid for Conrail, Norfolk Southern countered, and in 1997 the rivals split Conrail, with CSX taking about 42% and Norfolk Southern 58%, while Conrail Shared Assets kept serving Detroit, Philadelphia and northern New Jersey.

CSX now runs about 21,000 miles of track. In 2017 Hunter Harrison, backed by the hedge fund Mantle Ridge, became CEO and imposed precision scheduled railroading, with fewer, longer trains and deep staff cuts, before dying months later. In August 2025 Ancora Holdings launched a campaign against CEO Joe Hinrichs, citing an operating ratio that rose from 58% to about 67% and a 22% drop in third quarter profit. Management cited a weak coal market, Hurricane Helene damage, a Baltimore tunnel project and a $164 million goodwill impairment. Hinrichs was ousted in September 2025 and replaced by former GE executive Steve Angel, and in December the company disbanded its aviation department.

• The Juice Train hauled Tropicana orange juice from Bradenton, Florida to Jersey City, New Jersey.
• CSX rebuilds 1970s EMD GP38-2 locomotives with Wabtec, adding positive train control and new cabs.
• The 2001 CSX 8888 runaway pulled 47 cars unmanned for more than 60 miles and inspired the film Unstoppable.
• A December 2025 derailment in Todd County, Kentucky released molten sulfur and forced a shelter-in-place in Trenton.
• Since 1995 CSX has banned steam locomotives from its tracks, requiring $200 million in insurance from operators.