Pontifex is the official sequel to a freeware game in which you design and
test bridges. Pontifex uses a complex physics engine called the Immortal
Engine which allows for many different types of bridges to be built. Once
the design is completed you can test your bridge by sending a train across
and watch as the train either passes over safely or plummets into the river
below. The 3D engine lets you view your bridge from any angle including a
first-person train view. Many different types of levels are included, from
simple to complex, and a level editor lets you make your own levels to trade
with other people.
http://chroniclogic.com/pfx.html