The Travel tubes provide rapid transit between sections of the base and to the launch pads. The travel tube is evidently not a vacuum, as the creatures in Space Warp and The Beta Cloud use it without causing decompression. In Year 1 there are six seats (the Toga chair); in Year 2 there are four.
Access to and from corridors is via the double side doors. Access to spaceships on the launch pad is through the end door (which has straight sides on the set and angled on the model). This opens into the Boarding Tube.
The opposite end of the travel unit has no interior door. Instead, there is a communications panel with two black and white screens. Externally, we never see an end without a door. Arguably this is a continuity mistake in many episodes, but we can presume that the travel unit has a symmetrical outer shell for ease of manufacturing, and one door is non-functional when the interior is fitted.
When travelling, the forward side is illuminated with white light; the rear side with red light.