Trains are still highly efficient ways of quickly moving lots of people in/out/between cities. They're nearly as good for moving heavy, bulky loads between fixed points - like fuel for power stations.Which is why trains sort of almost work for the South East, assuming you want to go to London.
They really don't work anywhere else, and outside of London's Tube system, are not financially viable to users or the taxpayer.
The train companies often don't help, as parking at Chiltern stations is now a tenner a day, and Banbury station has closed it's drop off point to private vehicles, so anyone wanting to use the train from there now has to use a taxi if not in walking distance. Fine for drop off is now £40. So everyone drives to London now.