Orphan trains were a social experiment which transported the unwanted child and street kids from New York City, Boston, Chicago, and other crowded Northeastern cities to the Midwest U.S. for adoption. The genealogy of many of these 200,000 orphaned and abandoned kids who were moved to other 47 states between 1854 and 1929 can often be traced back to the Children's Aid Society, or the New York Foundling Hospital.
This is something I found quite amazing and almost never mentioned. For interesting reading and stories from the children (and of course, records -- perhaps of your missing relative), Google: "Orphan Trains."
Dee Notaro is an amateur genealogist based in Atlanta, Georgia (U.S.A.). Her own ancestral background is made up of a mixture of cultures, including her Irish forebears who hailed from County Sligo. Dee teaches classes on genealogical research and is passionate about helping others find answers to their ancestry questions.