John,
I'll be glad to take you up on that lunch idea, when I see you.
My museum has a multi-page Scope of Collections which details just what and what we do not collect. smaller groups simply have a Mission Statement. those are things you can ask for in your e-mail.
Also, a certin amount of common sense will work. Do not offer Pennsylvania Railroad items to the California Sate Railroad Museum and vice versa. Do not offer Central Pacific Railroad items to the Railroad Museum of Pennsylvania.
Maybe some reader can chime in on this; is the National Railroad Museum in Green Bay,WI really interested in everything?