Train puzzles usually consist of a specific track layout, a set of initial conditions (typically the starting place of each item of rolling stock), a defined goal (the finishing place of each rolling stock item), and rules which must be obeyed while performing the shunting operations. There are often constraints such as making the minimum number of couplings and uncoupling's, or making the minimum number of junction direction changes, or completing the puzzle. Other important factors may include the lengths of tracks limiting the number of rolling stock vehicles which can be placed along them.