How would it avoid that? It'd still be mega expensive to purchase plots of land in densely developed land (aka: cities) so your boring machine can tunnel up and

How would it avoid that? It'd still be mega expensive to purchase plots of land in densely developed land (aka: cities) so your boring machine can tunnel up and down from these station points. One would also need to be careful to not dig the upwards and downwards slope on existing infrastructure: pipes, underground electric and communications lines, etc. Porpoising doesn't help with any of that, if you need a slope of less than 2-3% inclination the machine will pass through a lot of potential infrastructure to surface and then dig down again. It doesn't make sense it'd save that much money considering that the expensive part of a project in developed land is that the land has a massive premium on it by being so productive.