The Carter Library is about a 20 minute walk from the Inman Park-Reynoldstown station on the Marta Blue/Green (east-west) line, quite a bit of it on a pleasant bike/ped path. Bus from the Amtrak station to the Red/Gold (north-south) Marta line, change at Five Points for the Blue/Green to Inman Park, walk to library. Would take about an hour total, depending on wait times.The Carter library at least has bus service.The Reagan library is at the other end of Simi Valley from the Amtrak station, and it appears that there's no public transit between the two -- which I think is exactly what you'd expect from the Reagan library.
No one at the Carter Center has any idea where the bus stops are, but, still, it exists....
Wish that damn Atlanta Amtrak station was downtown.