Frank:
That two-track version of the original pre-war **** GateBridge was designed by Lionel in 1999 so it was intended for the only Lionel track available at that time - tubular track. So it predates Lionel Fast Track and I’m not sure if Real Trax was on the market back then but I doubt if Lionel would have tried to make it compatible with Real Trax anyway. You have less than 6” between those two “curbs” to fit two tracks so you need something without a built-in road bed which ads extra width.
I have the green & yellow #6-32904 version and use Gargraves O gauge track across it and two tracks fit fine. A 3’ section of Gargraves Flex track overlaps the entire bridge so that you don’t have to fasten the track to the bridge, you just fasten it to the supports on each end. The width of the Gargraves ties gives a center-rail to center-rail spacing of about 3 5/8”. While that spacing is adequate for clearance on straight track, depending on what you are running, it will not likely be enough on curves. For reference, Lionel’s FasTrack curves come in 9” diameter increments which give a center-rail to center-rail spacing of 4½” when nested. While that should be adequate for most 3-rail O gauge rolling stock, I’m sure there are exceptions.
Like you, I took my bridge out of storage after 13 years of it sitting in its original shipping carton and put it on my layout this past year. I was surprised that it has lights in the towers which add a nice touch especially when the room lights are off at night. The original pre-war version doesn’t have those lights.
HTH,
Bill