In addition, carrying a concealed weapon onto the MARTA system is a felony, and no court is going to hold that an officer violated any constitutional right by pointing a gun at an armed felon.
Carrying a firearm onto MARTA grounds is not a felony, unless you do not have a GFL or are not a law enforcement officer, or other excluded persons listed in the firearm laws here in Georgia. It is a misdemeanor if you are caught carrying without a GFL to a "public gathering", the definition of which is totally ambiguous.