Hi Jeff,
Curved viaducts can be built, it just takes a bit of planning, a shedload of plasticard and a lot of time, but the results are worth it...
This is one end of our Stockburn layout; 6ft Radius, 18" high, 7' (ish) long
(the rest of the layout features a station at the other end and a coliery exchange yard in the middle. It's not often you get a busy double track main line and non-stop shunting at the same time on an exhibition layout! - apologies for the shameless plug !!)
Al