In Martian Dreams the system could only be accessed after getting into Olympus and repairing the power grid. Initially, Argyre was not available thanks to Raxachk blocking off the city. Only after forcing the way into Argyre did it become available again.
While they seem to have been related to pneumatic tube technology, such as we find at modern drive-through bank windows, I am unclear as to their exact structure and design. A shame. They were quite efficient.