Fans of Roller Coaster Tycoon would do well to consider entertaining themselves by playing OpenTTD, a community reverse-engineering / rewriting / extension effort based off Transport Tycoon (Deluxe), which was in turn the predecessor of Roller Coaster Tycoon. (RCT was originally going to be Transport Tycoon 2, but when Chris Sawyer tried out the new track-layout features the idea of roller coasters became obvious, and the game was history.)

Of course, this unusual legacy has some interesting side-effects and it can be a little weird to get into the game (e.g. there's two completely different models of doing signaling, and there's a complete rewrite of the passenger-traffic distribution algorithm that makes things a lot more challenging and fun but is buried in some sub-sub-submenu in the settings' Advanced Mode...)

Think of something a little like Roller Coaster Tycoon, but you get to do more network-design work and make tradeoffs between the expense of building a system right the first time or doing something more scalable for the future, and then seeing it blow up in your face when you try to run too much traffic down a congested line :D