A large proportion of our work has been to special requirements, such as this 1:8 scale steel static display bridge - a favourite project that went to Istanbul. It incorporated 7 1/4 inch gauge track parts from The Miniature Railway Supply Co.
The most economical approach to a custom design is to base it on standard parts. The picture below shows the first of many specials; this one for a large 0n30 railway, using mostly modified standard parts.
Masonry viaducts appearing to be curved are common, but there was rarely a horizontally curved steel bridge until the development of modern welded box girders. Most bridges that appear to be curved are a series of straight sections, and this is how we build a curved bridge. For a sharply curved bridge, the girders will be trapezoidal in plan, as in the example for an 0n30 railway shown here: