Hi,
You have a few of options. One is to make a cookie cutter sub-roadbed following the track run and build piers from wood.(see this build)
Another is to use a FasTrack to O tubular transition section and elevate a track like Atlas, Gargraves or even Lionel O.
The best look that I like for elevating the FasTrack only is using the MTH stone piers.40-1113 and 40-1134.
While there were some areas the railroads actually used elevated roadbeds with ballast, they were usually short sections. Elevating FasTrack definitely loses a prototypical look.