National Model Railroad Association
http://www.nmra.org/
National Model Railroad Association South Mountain Division
http://www.smdnmra.org/
Model Railroad Layout Design SIG
http://www.ldsig.org/
Operations Special Interest Group (OPSIG)
http://www.opsig.org/index.shtml
Train Collectors Association (TCA)
http://www.traincollectors.org/