Enables train stops based off priority. P is the priority, must be unique. Higher values have more priority, max is 29. L is the limit of the train stop. Supply 1 Green Signal when you want the stop to be enabled, as in the example (top combinator). Connect all stops together via the red wire on the big electric pole. The red signal is used to communicate priority, but this can be changed to have independant priority networks (change all red signals in the blueprint to another (unused!) signal)