This is what happens when a truss bridge and a girder bridge have too much to drink and spend a romantic night together.
My guess is the girder needed strengthening for some reason or another, and the railroad added the tie-bars and struts to give the girders more strength. It appears this either happened during original construction of the span or not very long after. The steel fabrication methods are similar, ie: rivets and pins. It does not appear this is a modern addition to the girders, but was done a while ago.
It may have been the original design for this bridge as well, if you look at the girders there are doubled up bearing stiffeners above each of the vertical strut members. These could have been added later, but who knows for sure.
Thatís one theory, I would love to hear others.
The bracing is reminiscent of the truss rods that were used to reinforce wooden railroad cars.
I am very interested in comments from more experienced pontist's regarding this design.