The one you might be thinking of is in Green Bay Wisconsin.
However there are a few that have the best curated, largest collections in the United States are:
-The Illinois Railway Museum in Union, IL
-Steamtown in Scranton, PA.
-The B&O Museum in Baltimore
-RR Museum of Pennsylvania at Strasburg.
-Portola FRRHS in Portola, CA
-Western Railway Museum, Rio Vista, Ca (outside San Francisco 90+ miles)
-Orange Empire Ry Museum
Georgia State Railroad Museum was created in 1989.
The web address of the National Railroad Museum is: http://www.nationalrrmuseum.org
The phone number of the National Railroad Museum is: 920-437-7623.
The web address of the National Underground Railroad Museum is: http://www.freedomcenter.org
The phone number of the National Underground Railroad Museum is: 606-564-3200.
The address of the National Underground Railroad Museum is: Po Box 421, Maysville, KY 41056-0421
The web address of the California State Railroad Museum is: http://www.csrmf.org
The address of the California State Railroad Museum is: 125 I St, Sacramento, CA 95814
The address of the Arizona State Railroad Museum Foundation is: 204 W Railroad Ave, Williams, AZ 86046-2556
The phone number of the Nevada State Railroad Museum is: 702-486-5933.
The web address of the Bay State Model Railroad Museum is: http://www.bsmrm.org
The web address of the Golden State Model Railroad Museum is: http://www.gsmrm.org