I don't see why anyone would change. If you missed the train you were aiming for by less than ten minutes then yes catch the next one to a different city, change trains after a circa 10 minute wait then travel the 12/15 minutes to the correct city, but that would only be quicker and work at certain times, for most people, who've missed their train, while it would be frustrating you'd have to wait for the next train (which is what they have to do now).
There's a couple of scenarios here wrt trains on a 4*1tph pattern. In all of them, changing is a useful thing to do, but only on some journeys as others are left stuck as hourly - ie you have to wait an hour for the train after the next train if you've missed your train - far more frustrating!
Half-hourly to each city, 40-20 pattern on the northern branches.
00 Ebbw Vale - Newport
10 Abertillery - Cardiff
30 Abertillery - Newport
40 Ebbw Vale - Cardiff
Miss the hourly Ebbw Vale or Abertillery to Cardiff train, wait 20 minutes for the Newport train, change in the 'core' for the train 10 minutes behind, get to Cardiff half-an-hour after you hoped, but half-an-hour before if you'd waited next direct train. People will change. Miss the hourly Newport trains, and changing makes no difference. You can flip the southern cities here, so Newport benefits a change and Cardiff not, but I went with Cardiff.
Or we can have a 40-20 pattern at the cities and half-hourly northern branches:
00 Ebbw Vale - Newport
10 Abertillery - Cardiff
30 Ebbw Vale - Cardiff
40 Abertillery - Newport
Miss the direct Ebbw Vale train to city - wait 30 minutes for train to wrong city, change in the 'core' for the train 10 minutes behind, get to city 40 after you hoped, but 20 minutes before if you'd waited next direct train. Miss the direct train from Abertillery, and changing makes no difference. You can flip the northern branches here, so Abertillery benefits from a change and Ebbw Vale not, but I went with Ebbw Vale.
Or we can have even 15s in the core with even half-hours to the cities:
00 Ebbw Vale - Newport
15 Abertillery - Cardiff
30 Abertillery - Newport
45 Ebbw Vale - Cardiff
Miss the hourly Ebbw Vale or Abertillery to Cardiff train, wait 15 minutes for the Newport train, change in the 'core' for the train 15 minutes behind, get to Cardiff half-an-hour after you hoped, but half-an-hour before if you'd waited next direct train. People will change. Miss the hourly Newport trains, and changing makes no difference. You can flip the southern cities here, so Newport benefits a change and Cardiff not, but I went with Cardiff.
Or we can have even 15s in the core with even half-hours to the northern branches:
00 Ebbw Vale - Newport
15 Abertillery - Cardiff
30 Ebbw Vale - Cardiff
45 Abertillery - Newport
Miss the direct Ebbw Vale train to city - wait 30 minutes for train to wrong city, change in the 'core' for the train 15 minutes behind, get to city 45 minutes after you hoped, but 15 minutes before if you'd waited next direct train. Miss the direct train from Abertillery, and changing makes no difference. You can flip the northern branches here, so Abertillery benefits from a change and Ebbw Vale not, but I went with Ebbw Vale.
Or we can do the 2*2tph that I propose:
00 Ebbw Vale - Cardiff
10/15 Abertillery - Newport
30 Ebbw Vale - Cardiff
40/45 Abertillery - Newport
Going to the city served by direct train - half-hourly service. Going to the other city, then take the half-hourly train, change for the train 10/15/20 minutes behind, and arrive. Every journey from northern branch to city has two opportunities an hour - nothing is left hourly - creating a more useful service for passengers. Add in that it's half-hourly at every station (even if there's 10-20 or whatever along the 'core'), rather than 15-45 or 20-40 intervals at one end of the 'core', improving journeys from the 'core' to whichever branching end (north or cities) has the uneven service under a 4*1tph scenario.
And, with 2tph, rather than 4tph along the core, 2*1tph is better than 4*1tp2h. A 3tph scenario
might work as 3*1tph rather than 2tph+1tph if the services desired are 2tph to Newport and Ebbw Vale, 1tph to Cardiff and Abertillery, just because you don't want to remove the existing direct Ebbw Vale to Cardiff service.