That's Union Pacific locomotive 1996 leading the train. It's one of the railroad's "heritage fleet" of locomotives painted in schemes evoking predecessor roads that have since merged into UP. Each unit is numbered for the year it became a part of UP or a predecessor later merged into the larger road. UP 1996 wears a Southern Pacific scheme based on the "Black Widow" and "Daylight" paint schemes. No SP locomotive actually wore this exact livery.
I'm amazed that I managed to get this shot off at all, and pleased to see the SP heritage unit running on "home rails."