CSX milepost prefixes

Most CSX railroad lines are given lettered prefixes before milepost numbers. These prefixes are one to three letters long with the first letter usually refer to the former company. For instance, prefixes beginning with "S" were part of the Seaboard Air Line Railroad. Some of these company prefixes include the following:
Atlantic Coast Line Railroad.
Lines that were formerly part of the Atlantic Coast Line Railroad system are given milepost prefixes beginning with the letter A. For instance, milepost A790 is on the main line in Orlando, Florida, 1000 miles south of the beginning in Richmond, Virginia, and milepost AN587 is in Waycross, Georgia, 587 miles south of Richmond on the branch to Montgomery, Alabama (which splits near milepost 548 in Jesup). Three-letter prefixes are used for minor branches of these branches, or minor branches off the mainline - for instance ANA is the northernmost branch of AN.
Before the ACL merged into the Seaboard Coast Line Railroad in 1967, none of the prefixes had the initial "A". The letters "I" and "Q" were skipped, and double-lettered prefixes (like KK, which would be AKK post-1967) were not used.
A is given to the main line from Richmond, Virginia to Port of Tampa, Florida. It has been pieced together from many sections of original main lines and newer connections; the full line is still in use by CSX.
AA is the northernmost spur, running from A89/AA89 at Halifax, North Carolina southeast and south to AA178 in Kinston, North Carolina. It was all built as the Scotland Neck Branch of the Wilmington and Weldon Railroad.
Seaboard Air Line Railroad.
Lines that were formerly part of the Seaboard Air Line Railroad system are given milepost prefixes beginning with the letter S. The mainline is numbered similarly to the Atlantic Coast Line route with the exception of the prefix. Also, like the ACL, the SAL mainline ran from Richmond, Virginia to Tampa, Florida.
The S Line is today discontinuous and segments of the original line have been abandoned (notable abandoned segments are between Chester, Virginia and Norlina, North Carolina, and between Riceboro and Seals, Georgia. The Seaboard main line from Richmond to Tampa, heart of its 2600-mile system in 1900, had been built by the following companies: