The Southern Railway Presidents' files in the SRHA archive date to before the Southern was organized, the early years of railroading in the Southeast are well documented
The attached letter to Samuel Spencer is a response to a letter from thr Southern asking if the cutoff between Jesup and Folkston, GA will be in service for the 1902 Florida travel season. At the time the letter was written, the ACL did not control the Plant System and as it considered both the ACL and Southern as "friendly" connections for passenger and freight traffic into Florida.
Previously, when the expanding SAL acquired the FC&P, relations between the Seaboard and the Southern became very much "un-friendly" as the SAL was nor interested in "short hauling" Florida traffic that had been interchanged between the Southern and FC&P. After the Coast Line acquired the Plant System, the Southern and ACL developed a working relationship that included traffic rights on the ACL from Hardeeville, SC to Jacksonville, Fla that lasted until the SCL merger. (As the ACL was not interested in short hauling either, until the SCL merger, the Southern and ACL only interchanged traffic at Tifton, GA, never Jacksonville.)
(Henry Flagler was concerned that FEC traffic bridged to the Southern by the Plant System and FC&P would be stopped. Once the traffic rights were established, the Southern and FEC had a direct interchange in Jacksonville the FEC considered critical for fruit and vegtable traffic from Florida to the midwest. Sam Spencer had agreed to "let the Plant System be the Plant System" (to pre-use an argument from the Conrail era) and not attempt to take over the Plant System as long as it was friendly to both the ACL and Southern. Both Flagler and Spencer were "blindsided" when Plant's widow sold the property to the Coast Line. A letter from Flagler in the SRHA archives asks Spencer if he was secretly negotiating with Plant's widow before the ACL deal was announced.
Southern being supported by J.P. Morgan probably convinced the Coast Line that a good relationship with the SR was in its best interest. That relationship also let them "gang up" on the Seaboard as unnecessary.