I recently needed to replace a short (~2ft) curved bridge and wanted something that would look good, but was cheap, quick, easy, and different than any of my other bridges on the layout.
I know that wood warps and doesn't last (hence the replacement), and metal tends to rust, but styrofoam will be here long after we're all dead and gone.
I had remenants of a large sheet of 1" thick that I bought to package some speakers for shipping. I was able to lay the track on the sheet and trace around it on the styrofoam to the desired width for the deck. I cut this out with a utility knife. The cut edge doesn't need to be perfect. It is covered in the next step.
I then cut some styrofoam strips that were about ~1-3/4" x 1" and glued these around the sides so they stuck up ~3/4" from the "deck."
For supports I simply cut rectangles of the appropriate length and width, again using the 1" sheet. I glued these to the bottom of the deck in the appropriate areas. I then used a file and sand paper to add a nice curve to the bottom of the deck between the supports. (Two inch thick styrofoam would yield a more beautiful, prototypical curvature for the bottom while leaving more thickness in the middle for support.)
I then coated the structure with DryLock hydraulic cement. This goes on with a thick, milk shake consistency and you only have a few minutes to work with it so do small areas at a time. When it drys, it looks exactly like real cement and is totally waterproof (not that the styrofoam much cares).
I put the bridge in place and the track was a perfect fit. The ~3/4" "side rails" that stick up are perfect to hold the ballast in place but are well under any trains.
Sorry no pics of this one yet.