Getting rid of all resistance via a vacuum tube makes theoretical sense but I don't see how it would be cost effective to say nothing of the obvious safety concerns.
What we also have to remember thou is that much of resistance is not only air but the track itself. This is one of the reasons why traditional HSR has such high operational costs........the wear on the wheels and the resulting wear on the carriage itself. This requires replacements and high maintenance costs.
This is where levitation shows it's strength. There are no wheels so there is no degradation resulting in much longer lifespan of the vehicles themselves, vastly lower electrical costs, a much smoother and quieter ride, and higher speeds........the trains have no moving parts.
The problem is the new track required but those costs are declining fast and especially in lightweight smaller vehicles. An excellent technology is a home grown one in Edmonton.
www.magnovate.com just got a deal with the Toronto Zoo for it's new trains and it's not going to cost the Zoo a cent. This is how cheap the system is to build an yet has potential top speeds of 500km/hr.
This is to say nothing of the fact that monorails are the safest form of rail due to being grade separated and most of all, due to having a "wrap around" on one rail, derailments are almost impossible.