OpenTTD (2004)

The free, open-source remake of Chris Sawyer's Transport Tycoon Deluxe has been developed by volunteers for two decades and has gone far beyond the original. Building busy main lines with path signals, presignals and grade-separated junctions is the heart of it, and the first time dozens of freight trains thread a crowded junction without stopping, you will see why people still play it. It costs nothing, which makes it the easiest place to start.