Most transportation in the 20^th Century was not sustainable, it was almost all powered by fossil fuels and emitted Carbon and Nitrous Oxides which are greenhouse gasses that are causing global warming.
Sustainable transport is transportation that produces no emissions that add to global warming in use (there will always be some in manufacture of transport vehicles).
Examples are Electric Vehicles, bicycles, Electric trains, E-scooters and walking.
All transportation needs to change to sustainable transportation