Thank you, updated and corrected.
This bridge does not belong to the Union Pacific Railroad. It is on the Burlington Northern Santa Fe Railroad, Stockton Subdivision, which runs between Stockton and Richmond. Both Amtrak and BNSF freights use the bridge several times a day.
This bridge is an extremely rare example of an Abt Bascule type of bridge (as patented by Hugo A.F. Abt in 1921), although I wonder if the plate girder portion is original and if it might instead have once had a truss there. Looks like the approaches have been altered, and maybe they put the plate girder portion in too? If the plate girder is original, then it would be the first non-truss example of this type I am aware of.
Regardless, the portion of the bridge with the counterweight and the a-frame like assembly clearly displays all the key features of an Abt bascule bridge.
The Abt bascule is a fascinating design which was invented and patented for the purpose of having a bridge type that could replace a bridge on existing alignment without disruption of railroad traffic. The Abt Bascule is sometimes called the American Bridge bascule because they apparently built many of them.
My Port Huron Railroad Bridge page discusses Abt bascules with links to two more in Michigan. I also have copies of the Canadian and American patents for the bridge http://www.historicbridges.org/truss/phrailb/index.htm
I agree with you regarding the visible A-frame structure but I am not familiar enough with this type of lift structure to know what portions are stationary and what portions move during the lift. More research.
The description is somewhat accurate, but I see what appears to be a Waddell type A-frame through truss here. Whatever you want to call it, a unique structure it is. I was looking at the surrounding area and there are a lot of interesting bridges in the area, including a large number of swing bridges.
I seek assistance in describing the style of this bascule bridge. There are three of these bridges on the UP BNSF rail line between Stockton and Brentwood, CA, across the delta of the San Joaquin river. This is the closest I could get to one of them.