Advtrains signals masts have full collisionbox but they occupy only a
small space. Also the collisionbox consists of two cuboids, one tall and
one flat. The flat one seems to be empty.
The selection box is supposed to look this way. The bottom centered "cube" is there so that you can rotate the signal without moving the cursor all the time, and to make stacking up easier.
However, the collision box is not supposed to work this way. I need to fix this.