Switches are a kind of route. You can build on any routes at an obtuse angle, but not acute. In other words, you can do AB or AC, but not BC.
In Australia, there are super switches. You build on one half of the route you plan to build, then roll a die and going clockwise, see where the trains take you. YOU MUST PUT THEM DOWN EVEN IF IT SENDS YOU IN THE WRONG DIRECTION, unless it leaves you with 2 train cars left, then you may refuse and try next turn.