For a fun little sandbox experience,
Train Frontier Express could be exactly what you desire.

INDIEGAMES.COM
A remarkably fully featured offering.
EUROGAMER
A nice break from all the side-scrolling Indie games.
KOTAKU XBLIG Pick

BUILD, RIDE, SHARE!

You can BUILD, RIDE, SHARE with friends in a this landscape building and rail riding good time.
Sculpt your own landscapes, then HOP ONLINE to share your map, take a ride, or build together in REALTIME.
Train Frontier is both STYLIZED and FRIENDLY, taking inspiration from MODEL TRAIN SETS, while eschewing the more complex train sim format.

TEAM TRAIN FRONTIER

The legendary model-train-simulator-game-making-collaboration:
EMOKS "emoks"
Art Wizard, etc.
Heavy metal Bob Ross

ERIC "xptnd"
Code Hippie, etc.
Grass-fed indie ruminator

THEIR STORY
The Two E's were happily sharing an office room at a totally legit AAA-game-studio, but were haunted everyday by the smell of a dead rat in the office-park false ceiling.
They longed for the day they would create indie games just like J. Blow, leaving behind the smell of decaying rodents in the walls of their totally legit workplace.
"It smells like a small mammal keeled over in this mainstream video game development office," is something we could imagine them saying.
Soon after, they were inspired to create the Greatest Model-Train-Set Simulator of late 2011.
Eight months later, TFE launched on the ol' Xbox 360, during the 2011 Indie Games Summer Uprising promotion. It's received several significant updates, while growing a small community of railfans, young and old alike.

SHOUTS

glitch city la
studio kaizen
dhx josh howard
indie games uprising
ceiling rat (RIP)