This tutorial explains the use and creation of Origin Brushes. Origin Brushes are needed for any entity that rotates.
What is an origin brush?
In geometric terms, the origin is the point (0,0,0), an artificial point of orientation in space at which the axes intersect. An origin brush is a brush that has the ORIGIN texture on all sides of it. The origin brush defines the axis of rotation for a rotating entity. Without an origin brush, a rotating entity will rotate around the coordinates (0,0,0) in the map. It also defines how a train entitiy will place itself over the track.
OK, so how do I use it?
If you were to make a rotating brush, make an origin brush at the point you want your rotating entity to rotate about. The origin brush's shape is not very important really, since the needed entity will always revolve about the origin brush's center. Then select the origin brush and the brush you want to rotate and tie them (group) to the desired entity. After that, edit the entity's properties to specify the axis of rotation. On func_train and func_tracktrain, the origin brush determines how the train will position itself over the paths. It's generally a good idea to place the origin brush in the center of the train.
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