It's not hard at all to come up with speculative transit ideas with some high-level engineering guesstimates. It's sort of a hobby of a lot of people. People have even sketched out plans for this particular idea (partially evacuated tube transport).

Implementing them: yes, that's harder, and most transit ideas fall down at that stage. Implementing them in a way that comes anywhere near the original optimistic projections is even harder (see: maglev).