The Exit region for subworlds must be parented to your subworld point in order for the physics of it to have any meaning to the avatar while in subworld.
The problem with that is it will move with your subworld point. To keep that from happening, use the Filter Inherit component, and you can restrict it's movement (either on just one axis, or all 3 if you desire.).
This works great for all sorts of things like elevators and rides. I talk about this in my tutorial:
http://dusty.homeunix.net/wiki/Making_S ... d_Rides%29You can use a responder to turn off and on subworlds, but I do not recommend you do it that way, as it's only good for solo play. If you use it, and your age is going to be in a multiplayer environment, it will cause all the other players in the age to fall through the floor that are not inside the subworld region.
You can also control subworlds with Python, if you open up and take a look at the python file for Teledahn's 3 floor elevator.
To keep people from falling through as you mentioned, you can use a "blocker" like a invisible collider who's physics you can turn off and on, depending on the state of your "ride". Set up a SDL varible that indicates where the ride is (you can either use a BOOL toggle for this so the states are 0 or 1, your you can use a SDL BYTE line and actually set the floor / station, etc that the ride is at. Then attach a python component somewhere and you can use xAgeSDLBoolResponder python file (if it's Bool, you have to use INT version if it's BYTE), and tell Plasma to either enable or disable your "blocker" based upon where your Ride is, have it "Disabled" when the ride is there, and "Enabled" when it's not.).
Of course some people do not like the idea of invisible blockers (like in Teledahn, when the elevator is not there, you just see this gap there, but you can't walk through it). In that case you could put in a animated door / gate / whatever that is rendered and will block the player (you have it animate based upon the "state" of the ride just as I mentioned above).
Again, there are several ways to do this and they work great.