WS says to use risers under all the track, even if no crossover or bridge on the layout. but all plans i can find that talk about it is HO, not N scale. if i dont use anything under the track but the base and the WS roadbed it will be alfull flat. if you put all the track up off the base, then you can make low spots, for whatever you want, pond, valley, whatever. you can do it with out going below the base/table.
if i was to put up a bridge, id use the inclines on top of the risers to get to whatever hight i need to clear whatever im going above,
i guess i dont know how to explain it.
look at a set or tracks in the real world, there not flat with the ground around them all the time, some are just a few feet higher then the ground around, (the hight of the roadbed), and some sopts are 10 to 15 feet higher. like theres a low spot in the ground , maybe the low spot lasts for a mile or more, to keep the track level, or atleast to keep the track from having a big dip, or to steep of grade, they build up the ground. like a damn on a farm pond, steep bank on both sides of the track/roadbed.