In Mortal Engines, the Sixty Minute War devastated civilisation as we know it, leaving most 'old tech' completely inoperational. Yet, despite the collapse of technology as we know it, they managed to do something that literally could not be done today: put cities (most astonishingly, London!) on giant traction treads.
In other words, the paradox is:
Today: Lots of resources and tech. Literally could not do traction cities if we tried.
Traction Era: Minimal resources and tech. Has traction cities.
Is there an in-universe* explanation for this?
*(The out of universe explanation is obviously rule-of-cool)