I want to add a Girder Bridge to my layout. I'm using woodland scenics track bed. I'm assuming that I put the track bed and the track over the bridge. My question is do you normally put railroad ballast on the bridge.
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We had both open deck and ballasted deck girder bridges on the NYSW when I worked there.
It depends on what you want, but if you are building a ballasted deck, you can use regular track (Gargraves, Ross, Atlas, Lionel, etc.) and continue the ballast straight across as if no bridge was present.
If you want a realistic open deck bridge, you'll need trestle track, which iirc, is exclusively Gargraves. It has longer ties, and guardrails built into the track bed. It doesn't have any cap timbers (the wood that runs longitudinally along the edge of the timbers/ties), but you can glue in some strip wood if you want that detail.
The trestle track comes in 37" sections, just like regular Gargraves track. As it is wider than regular track, make sure your bridge is wide enough to accept it.
This is the Bridge am putting on the board. I'm planing on Just a small creek to run under it. I'm using a 2" x 4'x 8' piece of foam with masonite on top. The lay out is flat.
That looks like a prime candidate for ballasted deck. I would suggest that you do indeed extend the track bed over the bridge.
Most bridges I've seen over water are the open-deck variety, but the PRR had ballasted ones over the Elizabeth River, and two over the Rahway River in New Jersey. Amtrak/NJ Transit still goes over them, and you can see them on Google Maps.
Also, are you putting the foam over the masonite, or vice-versa?
I already glued the masonite to the foam. This is a Christmas layout and will put away every year.
I have to carry it down steps , so I though that the masonite would make it a little more rigid and give the screws something to bite into. I thought about just flipping it over. But I need to cut channels for the wiring.
Early layout construction. A small open top box, done with pieces of plywood, 4" lower, allowed for the creek detail. Rock molds, and finish detail were done on the work bench, then installed, as one piece, in the layout opening pictured. 
WOW! That is nice.
