This was the transition period between steam and diesel, so late steam locomotives and early 1st and 2nd generation diesels would be what you would see, depending on the particular railroad. In general, steam was being/was phased out in the early/mid-1950's, again depending on the road.
The other thing to be aware of when you say "Southern" and "Seaboard" is the question of what roads were actually in existence via mergers, etc. then. I am not familiar with those roads, but you can try looking for articles on "fallen flag" roads, and when they might have merged to become something else. In my case, I model the Burlington Route in the same time period (actually 1940's to 1960). There were two subsidiaries, that utilized some of the same equipment, the Colorado & Southern and the Ft. Worth & Denver City. In addition, the Great Northern and Northern Pacific jointly owned the C.B.&Q. (Burlington Route), and pooled their motive power between Chicago and the Twin Cities on their passenger trains. They all were finally merged into the BN, but not until 1969, followed by acquisition of the Denver & Rio Grande Western and Santa Fe into what is now the BNSF. So you need to check the histories of the roads you want to model, and then check when they phased out their steamers.
Happy Thanksgiving!