Interlocking is like real signal box route setting. It can only be used in signalled areas. Only one train at a time can be on a selected route.

1. Add an interlocking tower object
2. Edit properties (if no properties, you have the wrong object)
3. Adding routes will show the path of the route.
4. You must path all intended routes.
5. To stop the path showing, click on hide paths button, but don't select the tick to close the box - just hit the X button (took me ages to figure that out).