There&#x27;s also the Vactrain concept.  Several variants were pursued from 1914 on, with Robert Salter of RAND writing a couple of proposals in the 1970s.<p><a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Vactrain" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Vactrain</a><p>Salter, Robert M. (August 1972), The Very High Speed Transit System, RAND Corporation<p><a href="https:&#x2F;&#x2F;www.rand.org&#x2F;pubs&#x2F;papers&#x2F;P4874.html" rel="nofollow">https:&#x2F;&#x2F;www.rand.org&#x2F;pubs&#x2F;papers&#x2F;P4874.html</a><p>Salter, Robert M. (February 1978), Trans-Planetary Subway Systems: A Burgeoning Capability, RAND Corporation<p><a href="https:&#x2F;&#x2F;www.rand.org&#x2F;pubs&#x2F;papers&#x2F;P6092.html" rel="nofollow">https:&#x2F;&#x2F;www.rand.org&#x2F;pubs&#x2F;papers&#x2F;P6092.html</a>