A tunnel-tube junction station was a large box-shaped unit that was primarily used at construction sites on airless planets. They were in use until a suitable underground tunnel system could be constructed. It was mounted on a tread system, for mobility, and housed four retractable tunnel-tubes. The station, with its self-propulsion unit, could be moved into position and the tubes extended to dock with another junction station, starship, or the hatch of an erected building . They also contained viewports and the main control console. The console controlled the propulsion unit and steering controls, also, commo and safety/alarm circuitry, along with lighting apparatus. Gravity-control systems and life support were also managed from the console. The junction station and tubes sometimes emplo