This graphic shows the line turning west at Golden and heading up Clear Creek Canyon. Let's imagine cramming this into that scenic canyon: two tubes (because you have to allow return trips), or three tubes (because things need maintenance, and the third tube could be reversed for high demand). Each tube must be at least 12 feet wide inside for comfortable four-across seating. So you have a bundle of tubes at least 25-30 feet thick if stacked, or 40 feet wide if side by side. Now route that through a narrow, winding canyon beloved for its scenery. Add maintenance facilities with the massive air pumps to maintain the vacuum. And please don't disturb the current US 6 road. 

Would you really want to sacrifice the canyon for this? For a "high-speed" line that couldn't possibly negotiate those curves at high speed?