Clark,when you all mentioned about steel encased in concrete i remember seeing up close and personal a steel beam encased in concrete on a bridge like this that a short line operates over in Bucks County just south of Quakertown Pa.Me and my fiancee both saw it and i took a picture of it.The hole was pretty big.Don't know if they sealed it back up or not.Funny how you can talk about something and then remember it.
That's what I was thinking may have been done here Clark.
I've noticed, around western Missouri at least, that some rail bridges were made with steel stringers spanning the abutments, then encased completely in concrete.
It's hard to say whether this would be considered well protected stringers or a heavily reinforced slab.
Definitely not a steel stringer, but not a concrete rigid frame either--I would probably call this a slab.
NBI calls this a steel stringer.