Also, Musk explicitly said it's not that ("But Elon has said there’s no vaccuum tubes involved").
The bad effects of air-friction would seem to still be present, and maybe much greater, because you'd have to move a huge mass of air and the friction would be over a much greater surface area (isolating the moving air to within a small portion of the tube does not look simple to me).
f=0.006; %wall friction factor inside tube, also skin friction outside a train rho=1.2; %air density [kg/m^3] R=1.5; %tube radius [m] Vg=0:200; %velocity of tube gas [m/s] Vt=250; %velocity of train [m/s] L=500e3; %length of tube [m] Cd=0.8; %drag coefficient, form drag trainfill=0.1; %percent of tube filled by train airfill=1-trainfill; %percent of tube filled by air ncars=1000; %number of cars in tube Pg=pi/4fVg.^3LairfillR; %power required to move gas through tube Pt_fm=ncars0.5rhopiR^2Cd(Vt-Vg).^3; %power required to overcome form drag on each car Pt_sk=pi/4f(Vt-Vg).^3LtrainfillR; %power required to overcome skin friction drag Pg_annulus=0; %since the mass of air is so small between the train and tube wall, assume 0. Pt=Pg+Pt_fm+Pt_sk; %total power requirements