Which subtheme did the "Space Monorail" sets come under? Many years ago I remeber attending a model railway exhibition and one of the railways was a Lego monorial that looked very "space" based. I've found 6990 (Monorail Transport System) and 6991 (Monorail Transport Base) but I can't find out what sub-theme they came under.
BrickSet lists 6990 under the Futuron space sub-theme and 6991 in the Unitron space sub-themes.

There was also a Monorail sub-theme of City.