Various methods can be used to construct viaducts and bridges for a model railroad layout. This short video walks you through the construction of a model bridge built from corflute and cardboard.

The plans for this bridge can be adapted without too much effort, for example, the arched portal can be made to be a single or double portal (for two tracks). The arched portal can even be blocked off thereby turning the portal into a sealed brick archway.

And, very importantly this bridge can be increased in length or made to be wider, depending on what is needed on a particular layout. Just one set of plans can be purchased and if a mistake is made or want to extend the length or width of the bridge, it is simply a case of printing out an extra copy. This tutorial demo shows the plan being constructed to OO scale size, however, this bridge can be made to N scale size, or to HO scale size which is 87% of OO.

You can see more details at https://www.modelbuildings.org/bridges-and-tunnels-pack-deal-c/