Pontifex

In Pontifex you design and test bridges. Pontifex uses a complex physics engine which allows the construction of many different types of bridges. Once your design is completed, you can test the strength of your bridge by sending a train across it. Depending on the quality of the bridge's design, the train will either pass over safely or plummet into the river below. The 3D engine lets you view your bridge from any angle, including a first-person "strapped to the front of the train" view. Many different levels are included, from simple to complex, and a level editor allows you to make your own levels to trade with others.

Source: The Developer Website.