Considering electric planes aren't feasible yet, how else do you plan to decarbonize airplanes?

Short range electric planes are being manufactured presently.

This wont be able to decarbonize anything until surplus carbon free energy is generated for it use - that need not take decades with proper governmental policies, but it means this technology will not lead the way.

Replace, as far as practicable, with fast trains and maybe Hyperloops.

How do you figure?

Estimate the cost of the energy inputs per gallon of gasoline. Account for the inefficiency and you get $15-20/gal.