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
I play a bit of OpenTTD from time to time. There's quite a learning curve, but it's enjoyable. For all the tweaks though, there's a couple that I always want, are always the source of me stopping play:
I don't like negotiating with towns to let me lay down roads and other infrastructure. After a while dealing with this nonsense by planting thousands of trees gets old and doesn't work all the time. I wish I could just turn that off.
I wish resources didn't run out. By the time I have a pretty cool multi-mode transport network worked out, somebody will run out of resources and I have to undo the whole thing. The maps are big enough that infinite resources would still be fun.
The game gets into these weird modes were my boats or trains or whatever are shuttling around tons of mail, or the lines to use service at one stop are unbelievably long and there's no activity anywhere else. I think it's a bug (but I'm open to it being a problem in my transport network as well).
The GUI gets a little obtuse and fussy for my taste. It's okay when you're building up a mode of transport, but once your network starts to get really medium sized you quickly run into trouble trying to keep track of what's going where or doing what.
Shameless plug: The guys behind OpenTTD are also working on a remake of RCT called FreeRCT [1]. It's still in its infancy and it doesn't have many features, so any contributions are welcomed!
The developers behind OpenRCT2 call it open source but because it's a derivative work they don't actually have the ability to license it under anything.
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