[John Webb]

From a few photos in various books I have it would seem a short metal girder bridge, across say a stream or farm track, might well have ordinary ballasted track. But larger bridges mostly appear to have the large longitudinal timbers Brian describes. You can use ordinary track by gluing it down onto your timbers then cutting away the sleepers. A better representation can be got by using PECO track and their slide-on chairs and gluing these down to the timbers.
Regards,
John Webb