Hi. I would suggest some kind of upgrade/reward for creating a circular line in extreme mode. When creating a circle, that line is pretty much done for changes. Granted I acknowledge that the line will be more efficient, but you are locked to do any expansions. Because of this, there are rarely any reason for closing a line in this mode. Some suggestions:
All stations on a circular line would be upgraded to intersections
All “none circle” stations on a circular line would be upgraded to intersections
All trains on a circular line will get an extra carriage at the moment of completing the circle line.
Or something of the likes. Just to make it a strategy to consider circular lines