Paul,
The cross bracing that is on the joists originally had some beefy longituinal stringers that ran the length of the bridge, something like 8"w X 14"tall. Then the ties sat on those. This made for a very solid bridge.
Our modification, under the approval of the bridge engineer, is to fill in the spaces on top of the joists between the cross bracing. Then lay the track directly on the cross bracing and our new filler blocks. The 3 inch decking will then be laid parralell to the rail for the length of the bridge and approach spans.
This new design accomplishes 2 things. First, the lower height allows the bridge to be installed at current grade height, where to build it as it was would have required raising the roadbed about 2 feet.
Second, the 3 inch decking (was planned originally) will allow our contractor to access the south side of the bridge and make repairs to the grade and haul in material much quicker than we ever could.
Hope that answers your question.