i was just thinking that it would be nice to have a rail block that doesn't move entities along it but if a docked entity is pushed by another entity it would move (think the minecarts in minecraft). it might also be nice to have a half and half block that moves the docked entity but if it is pushed with enough force it will move in the other direction or faster in the same direction. this could be achieved with the original rail block and logic ie if there is an active not block connected...
free moving rail block
free moving rail block