There are several hundred railroad museums, tourist railroads and railroad history displays in the United States and Canada.
One of the best places to start is with the Tourist Railroad Association, Inc., "TRAIN", which is the trade association of recreational railroads and railroad museums.
Their website, TRAIN has a search engine and links to many of their members.
Check out the website of the Mid-Continent Railway Historical Society, an outstanding railroad museum which operates steam engines from Mid May until the end of October.
A good book which tells about all the sites in the United States is the Steam Passenger Directory. This book is published each year by the Empire State Railway Museum. The Railroads and Museums are sorted by State and Province. Schedule Information, Fares and Equipment are also listed. The book sells for around $14.95. It is currently marketed by Kalmbach Publishing's Trains Magazine
Their online catalog, The Kalmbach Catalog, will provide information on how to get the Directory.
Also, two of the leading magazines, Trains and Railfan and Railroad, list specially scheduled excursions and have a directory of Railroad Museums.