someone mentioned Carmarthen to Aberystwyth and there are a few other examples in Wales of journeys which were once easy by rail but are now very complicated. ....
Caerphilly - Pontypridd
Merthyr - Rhymney - Ebbw Vale (Heads of the Vallies line closed in 1958)
Coryton - Pontypridd (an extension of the line from Coryton existed until 1931.... I think)
Pwllheli - Bangor (via Afon Wen until 1964). Haven't bothered to work it out but shudder to think how long it would take today!!
These off the top of my head but sure there would be others.
Llandrindod to Newtown springs immediately to mind, since I was there a couple of weeks ago! There are plenty, though, as you say.
The line to Coryton did once extend through to Radyr. IIRC passenger services beyond Coryton did not survive after 1931, though the line remained for freight, mostly Valleys coal, until 1952. I could be wrong though, it's a long time since I read up about the line's history.
One of the bigest ironies is that trains now run from Coryton to Radyr, taking about 40 minutes, whereas the direct route would have been a lot quicker! Not that the trains exist to serve a huge Coryton - Radyr market, of course!
Suburb to Suburb Journeys can be particularly slow because it is often a 3 part journey. A journey I did a lot when I was working away from home illustrates this well, Altrincham to Hall Green (Birmingham). Even though the ends were not too far from stations there still had to be allowed another 10 minuets time at each end and the best that could be achieved was a little over 4 hours. In contrast by car on a typical day it could it could be done in about 2hours 15min (Best about 1h 50min) and the worst was 4h 30 min when the M6 was closed. And this was done in a £6000 60MPG Citroen C1 with a quoted running cost of around 30p/mile.
Indeed, and this is why public transport will always lack the flexibility of a car. With the latter it doesn't matter where you are starting out from or ending up at, you are not constrained by a fixed route or a timetable.
It's no wonder why cars remain as popular as ever!