The Network Addon Mod, or NAM, is a modification for SimCity 4 Deluxe (or SimCity 4 with the Rush Hour Expansion pack), which adds a myriad of new transport network items, ranging from ground light rail, to fractional-angle roads, to roundabouts, and much more. It also includes bugfixes to Maxis items and a highly optimized set of traffic simulator plugins, improving pathfinding accuracy and performance.
Its just great. Fun and challenging, you have to manage a budget while providing necessities and keep everyone happy. And if you get bored just download some stuff from the HUGE modding community plus its really easy to install any mod. Just find a free copy of WinRAR.