NY -- Standard&Poor's CreditWire 7/3/96 -- Standard&Poor's today has lowered its corporate credit rating on Union Pacific Corp. to triple `B' from single `A'-minus. At the same time, Standard&Poor's has raised its corporate credit rating on Southern Pacific Rail Corp. to triple `B' from double-`B'-plus. Standard&Poor's has removed these ratings from CreditWatch, where there were placed Aug. 4, 1995. The rating outlook on both companies is stable. Additionally, Standard&Poor's has affirmed its `A-2' commercial paper rating on Union Pacific, which was not on CreditWatch. (See list below for related rating changes.) The rating revisions reflect the announcement that the Surface Transportation Board (STB; successor to the Interstate Commerce Commission) in