Roger H. Bennett writes
Cross-London regional journeys are difficult to make at present, so
Crossrail, TL2K and more effective use of the Willesden Junc-Clapham
route will help that - although the latter route probably suffers
because it avoids central London.

I was thinking of intercity journeys but not necessarily intercity
trains. A regional train from Peterborough or Cambridge, with limited
stops, could provide a more attractive alternative than going to Kings
Cross, changing to the Tube, then changing again at another London
terminus.

Those are what are what is meant by 'regional' journeys.

--
Dave