Bingo-
Any type of track will work; it depends more on how it will be supported. Atlas, Lionel tubular, MTH scaletrax (non-flexible sections), and Gargraves (non-flexible sections) are fairly rigid, and as such do not need as many supports. Gargraves' flex trax, Lionel's Fastrack, and MTH's Realtrax are fairly flexible over long spans, and as such need additional support.
There is a huge variety of trestles on the market today, ranging from Lionel's 110 and 111 trestle sets to scale versions of subway trestles from manufacturers such as BridgeBoss and Landro Trains.
In the end I think the deciding factor might be how well you want the elevated line to "fit" with your existing layout. A toylike-looking trestle might look out of place on a hi-rail layout, whereas a finely detailed trestle might seem a bit too much on a postwar-themed layout. The important thing to remember is that no matter what you have to be happy with the end result, so with that in mind there can be no "wrong" way to add an elevated track to your layout.
Hope this helps!
-John
