Sure, there will still be cars and lorries about, but their usage will be more akin to that experienced in the age of rationing during the 1940's and 1950s. For the rest there will be public transport or trains.
Given the laughable state of the rail network and the enormous fare costs, this is only practical if people accept a drastic reduction in the quantity of goods they consume, including food and drink and everything else, and a drastic reduction in their personal mobility.
Frankly the rail network is far too expensive and totally unfit to bear a larger proportion of our transport needs, and expecting people to accept a return to the 1940s lifestyle is absurd.
Cut fares by three quarters if you want to build the world you imagine. And abolish idiotic railcards.
As for freight, moving any significant quantity of freight on the UK rail network would require essentially building an all new network - the existing one simply is not suitable in any real sense.
Trains are too short and too gauge restricted.
Essentially, with a level playing field that mandates zero emission from all surface transport within a decade or so, rail traffic will multiply to the level that - of the current network - there will be virtually nowhere left that does not justify wiring of some description.
It would be cheaper to embrace the Siemens eHighway than attempt to fix the rail system with it's current culture and ownership/operation structure
A perfect application for discontinuous techniques then.
A better option would be to note the relatively low traffic in and out of Marylebone and the close integration of the Chiltern infrastructure with London Underground and simply build the base of the Chiltern system as a third rail assembly.
But because of ORR intransigence.... we will get nothing instead.