Numerous named passenger trains were operated by the railroad for Florida-bound tourists, with the Atlantic Coast Line contributing significantly to Florida's economic development in the first half of the 20th century.

[5] An investor from Baltimore, William T. Walters, gained control of these separate railroads after the Civil War, and operated them as a network of independent companies.

Other small acquisitions took place in 1901, and in 1902 the ACL took over the Plant System, which operated numerous lines within Florida and Georgia.

However, the sprawling ACL system needed larger and more modern facilities to handle locomotive overhauls and freight car building.

By the 1920s the two largest shop sites were at South Rocky Mount, North Carolina and Waycross, Georgia, each of which employed about 2,000 workers.

[9] By the early 1900s the railroad had largely reached its final configuration and began to focus on upgrading its physical plant.

[3] The railroad provided a submarine-proof alternative to coastal shipping, and it also served the fast-emerging military industry in the Southeast.

[3] In 1942, Champion McDowell Davis (nicknamed "Champ") became president of the ACL after starting with the railroad in the 1890s as a messenger boy.

In 1956 the railroad moved its headquarters, which had been sited at and adjacent to Wilmington, North Carolina's Union Station to Jacksonville, Florida.

[12] As early as October 1958[4] the ACL and competitor Seaboard Air Line Railroad had discussed the possibility of a merger, initiating extensive studies on the potential unified system.

[15] During its early years, the ACL handled mostly seasonal agricultural products, but by World War II its freight traffic had become more diverse.

[3] During the 1950s, the ACL acquired some 13,000 new freight cars, to be used on high-speed trains offering reduced running times compared to earlier equipment.

Trains for Miami ran on the Florida East Coast Railway from Jacksonville to Miami, but after passenger service on the FEC effectively ended with a long-lasting strike in 1963, ACL transferred its Miami-bound trains to Seaboard rails at Auburndale, Florida.