Cube Trains is a free, open-source game about figuring out how to route rails through a dense urban environment. It offers a degree of freedom not usually found in a puzzle game, while retaining the challenge. The game offers a variety of difficulty levels, in addition to an easy-to-use level editor. Infinite undo/redo. Will make you feel clever.
Testing some new graphics. The tunnel design, here, didn't make the cut.