Track based play systems are described which comprise a set of physical play pieces and an associated computer game. In an embodiment, a user may arrange some or all of the play pieces in the set to form a path. The computer game is arranged to infer a virtual model of the path defined by the user-created arrangement of the play pieces. The inference may be based on data communicated by one or more the play pieces to the game or based on data from a local sensing device such as a camera which views the relative positions of the play pieces. Having inferred the path, the game constrains a virtual or physical object to the path within the game play and renders a graphical user interface showing at least a portion of the path.