I have been trying to determine what happened to my great grandfather, Daniel Cavanagh. After Civil War, he disappeared. Family lived in Rock Island. Some believed he died while working on the Union Pacific. Might UP Museum help me out?

Try Ancestry.com they have an amazing amount of information.

There is a genealogical library in Councils Bluff that you should contact

There are alot of general exhibits but no specfic information on individuals other than the founders

If he was a big deal with the company maybe?

Possibly but most likely not