NY -- Standard&Poor's CreditWire 5/8/96 -- Standard&Poor's today has assigned its double-`A'-minus rating to Union Pacific Railroad Co.'s $69.2 million pass-through certificates series 1996-A1 due 2002 and $103.0 million pass-through certificates series 1996-A2 due 2003, and places the issues on CreditWatch with negative implications. The corporate credit rating is single-`A'-minus (also on CreditWatch with negative implications). Union Pacific Railroad is a unit of Union Pacific Corp. The double-`A'-minus rating on the issues reflects special protection afforded the issues under Section 1168 of the U.S. Bankruptcy Code. The ratings of Union Pacific Corp. (UP) remain on CreditWatch with negative implications and ratings of Southern Pacific Rail Corp. (SPRC) remain on CreditWatch with positive implications, where they were