It is a powered rail that detects if a minecart chest on it is empty or full.
It has a normal and an inverted mode.
In normal mode it powers when the minecart chest is full (this would be for a loader - you would have a hopper above the minecart chest to load it), and in inverted mode it powers when the chest is empty (this would be for an unloader - you would have a hopper below the rail to unload the minecart chest).
It has a normal and an inverted mode.
In normal mode it powers when the minecart chest is full (this would be for a loader - you would have a hopper above the minecart chest to load it), and in inverted mode it powers when the chest is empty (this would be for an unloader - you would have a hopper below the rail to unload the minecart chest).