How to build bridge using 4.5/12v train rails from 1969 Lego brochure
Here is an approximation of the bridge that appears on the cover of the catalog pictured above. Unlike the version in the picture, this model uses bracket pieces to attach the arched section to the rail bed. All the pieces in this build were available prior to 1994.

The arches are decorative and do not really contribute to the strength of the bridge. This is completely computer-generated and may not be very sturdy in real life. It would not be difficult to adapt the rail bed to give it more strength, however.

I put together an LDraw file with steps for building this bridge. The file can be downloaded from my MOCpage.