CSX Transportation (reporting mark CSXT), known colloquially as simply CSX, is a Class I freight railroad company operating in the Eastern United States and the Canadian provinces of Ontario and Quebec. Operating about 21,000 route miles (34,000 km) of track, it is the leading subsidiary of CSX Corporation, a Fortune 500 company headquartered in Jacksonville, Florida.

CSX Corporation was formed in 1980 from the merger of Chessie System and Seaboard Coast Line Industries, two holding companies that controlled railroads operating in the Eastern United States. Initially only a holding company, the subsidiaries that made up CSX Corporation completed merging in 1987. CSX Transportation formally came into existence in 1986, as the successor of Seaboard System Railroad. In 1999, CSX Transportation acquired about half of Conrail in a joint purchase with competitor Norfolk Southern Railway. In 2022, it acquired Pan Am Railways, extending its reach into northern New England.

Norfolk Southern remains CSX's chief competitor; the two share a duopoly on transcontinental freight rail lines in the east half of the US.

CSX is the result of a number of mergers among railroads operating in the eastern United States, the earliest among them the Baltimore and Ohio Railroad (B&O) which formed in the 1820s. Many of the competing railroads along the east coast began merging from the 1950s onward as part of a broader trend of consolidation. An announcement from the New York Central (NYC) and Pennsylvania (PRR) railroads in November 1957 that they were considering combining set off discussions between the Baltimore and Ohio Railroad and the Chesapeake and Ohio Railway (C&O) on a merger. Ultimately, the financially stronger C&O took control of the B&O in December 1962, though the two railroads kept their separate identities. The NYC and PRR ultimately formed Penn Central Transportation Company in 1968, which by 1970 was bankrupt.

The combined C&O/B&O purchased stock in the Western Maryland Railway until it was able to take full control in February 1967, bringing a third railroad into the combined entity, which in 1973 became formally known as the Chessie System after the C&O's historic cat mascot Chessie.

While the railroads in Appalachia were merging, southern railroads (and historical competitors) Seaboard Air Line Railroad and Atlantic Coast Line Railroad decided to pursue a merger in 1960, which was authorized by the Interstate Commerce Commission in late 1963 and finally completed in 1967, forming the Seaboard Coast Line Railroad. The combined company absorbed the Piedmont and Northern Railway in 1969.

In the Midwest, the Louisville and Nashville Railroad (L&N) went on an acquisition spree, splitting the Chicago and Eastern Illinois Railroad (C&EI) with the Missouri Pacific Railroad in 1969. This was followed in 1971 with the acquisition of the Monon Railroad, which had complained bitterly about the C&EI split. The L&N also purchased a portion of the Tennessee Central Railway in 1969. While still independent, the L&N had long standing links to the Atlantic Coast Line, and other railroads in the region began to worry about a combined L&N/SCL system.

In 1969, the Seaboard Coast Line created Seaboard Coast Line Industries as a holding company. The Seaboard Coast Line Railroad had already held some of L&N's stock, but the new holding company began buying up as much as it could find and held nearly total control of shares by 1971. With this also came control of the Clinchfield Railroad and Georgia Railroad, both of which were nominally jointly owned by SCL and L&N. The resulting railroad conglomerate began operating under the name "Family Lines".