Steam, Tracks, Trouble & Riddles (S.T.T.&R.) is a single player adventure mod, based on source engine (Orange Box) by Valve. It combines two types of game play: an adventure game with complex, 3D-interactive riddles in a fantastic environment and a simulator for track based vehicles.
Spoiler Alert! Be aware that this can spoil your game experience! Guide to all resolutions of the riddles. Just in case you stick in the middle of the mod.