Is it true credit mobilier was a government agency that was established to use silver to back the currency of the US?

No, that is not true. Credit Mobilier was a private railroad construction company established in the 1860s, primarily known for its involvement in the Union Pacific Railroad and notorious for a scandal involving bribery and corruption. It was not a government agency and did not have a role in backing the U.S. currency with silver. The backing of U.S. currency with silver was related to the Coinage Act of 1792 and later debates around the Gold Standard and the use of bimetallism.