Twenty-two years after its release, SimCity 4's modding community is still going strong, with dedicated fans continuing to develop and share new content for the classic city-building game. The community's efforts have added features that were never implemented in the original game, including multilevel highway interchanges and intricate railroad networks, which can be seen in YouTube videos showcasing players' creations.

The modding community's work is a labor of love, with modders like Tarkus drawn to the game's deep and complex simulation, which remains unmatched by newer games like Cities: Skylines and its sequel. For others, like Ulisse Wolf, it's about the community itself, which he finds to be "more mature" and a "quiet place to live in peace with other members."

SimCity 4 was released in 2003, the fourth in game company Maxis' series of city-building games. The game's ability to allow players to add their own content was a key part of its appeal from the start, with a set of tools that allowed players to create and add their own buildings to their cities. Players soon began sharing their creations online and discussing tips and ideas in accompanying forums.

As players looked beyond the officially supported methods of adding content, they began to wonder what else could be done. Could the game's road system be expanded? What about the railways? Early modding efforts were made easier by the way the game stores its data, which is in a database whose format was shared with previous Maxis games. However, accessing this database required the development of a suite of tools, which the community eventually created.

As modders began to find their way around the database, they started to find entries for systems that weren't in the game, including features that were either unfinished or never implemented. Using the tools they'd developed, modders were able to build the rudiments of these unfinished features into entirely new ones. One of the first such features to be repurposed was an unused road type, which became the basis for the "real highway" mod, a more flexible and customizable alternative to the game's built-in highway system.

Today, the Network Addon Mod (NAM) is a sprawling package that includes the RHW mod along with a huge amount of other transport-based content, including pedestrian malls, a new railway system, roundabouts, canals, bridges, and underpasses, as well as a comprehensive reworking of the game's traffic simulator. The mod remains in active development, with its 49th version released in March 2025, and is maintained and developed by a constantly shifting and evolving team of around 25 full-fledged members, along with other part-time contributors, playtesters, and more.

The community is friendly and welcoming, with veteran members welcoming new fans and what Wolf calls "political bullshit" notably absent. It's not just the NAM that remains in active development, either. In 2025, SimCity 4 modding continues to evolve and encompass new possibilities, largely because of improved access to the game's source code.

In the 2010s, the release of a portion of the game's code, provided by the late Paul Pedriana, allowed for the reimplementation of some of the game's internal debugging features. Then, in 2019, a 64-bit macOS port of the game was released, which unexpectedly provided modders with access to certain possible hooks to work with. Modders continue to lack full access to the game's code, but even limited access has allowed the use of techniques that would have otherwise been impossible.

The ability to write custom DLLs has allowed modders to fix long-standing bugs in the game, including one notorious issue that could lead to save game corruption in large cities. It has also let them dream up features that would not have been possible otherwise, including a 3D camera, the reimplementation of the game's long-broken water system, and a Steam Workshop-esque interface for finding and downloading custom content.

Despite the challenges of working with a game that is over two decades old, the NAM team and its fellow modders remain committed to pushing the game's limits and exploring new possibilities. As Lucario Boricua puts it, working on the game today feels like "standing on the shoulders of giants," with the decades of work that modders have put into the game providing a platform for today's creators to push the game into places its designers probably never imagined.