A transfer line was a physical distribution mechanism utilized aboard 22nd century Starfleet vessels for the purpose of connecting the ship's central ventilation to the appropriate tank. The transfer lines could by rerouted to a different tank by recalibrating the switches on the control panel of the ship's atmospheric recycling manifold in sequence for the desired gas.

In 2152, Doctor Phlox was required to reroute the transfer lines to the ship's carbon dioxide tanks in order to render members of the crew unconscious who had been taken control of by a series of non-corporeal lifeforms known only as "Wisps," in order to evict the beings from the affected crews' bodies. (ENT: "The Crossing")