CrossCode is really good, but it isn’t free. I need to look through my Steam library to see which other games might be open source too. This is the only one I’m aware of, really. ProtonDB might have a list of games that are open source. It’s a great resource for checking compatibility if you’re playing on Linux.
OpenTTD is fantastic. The graphics might give new players pause, but if you like building networks or logistical puzzles, or just like trains, it’s still one of the best in the genre.
Probably not unless you get public funding (eg, run a massive deficit, take control of the public AI to build stuff, debug features, etc) or you first create a resource chain or two to pay for transit.
It’s definitely a work-in-progress title, but I have really enjoyed SuperTux Advance (AGPL-3.0 according to their github page). It’s like SuperTux but with sliding, more playable characters, and more power ups. As of now it’s probably my favorite OS game at the moment. -Ace