We also feel an opposing force on our bodies due to our own inertia, whenever we change tubes. Thus, space tubes are essentially inertial frames of reference that depend upon the uniform motion of the observers traveling in the spaceship. This is necessarily also the path of least resistance through that space. Moreover, if distant objects in the visual field appear so distant that there is no apparent relative motion observed at all, then again, the tube may be equivalent to a dome and motion again becomes undefined. In agreement with Mach and Einstein, motion is meaningless where we have no objects for direct comparison.
Linear motion is, therefore, locally defined.
Otherwise, linear motion becomes undefined. One cannot precisely define linear motion outside of one's own frame of reference. In this case, the tube is still defined as described above. Hence, in this case, there can be a tube as defined above and this is equivalent to our forward motion. There can be no distinction here.