Fastrack ties are nearly scale while Gargraves ties are greatly oversized. While not a problem visually when using either track I think putting the two together would clash, especially since it's a bridge and you won't be able to use ballast to blend the two.
You bring up an interesting point here: how can Fastrack users have bridges without ballast?
I am using a Lionel Fastrack trestle set and will (#getaroundtoit) conceal the Fastrack ballast with horizontal timbers to mimic a bridge deck but that will be kind of simplistic and schematic.
Lew
