> The train to Bristol & Cardiff passes Heathrow just a few miles away, but there is no direct rail connection at all.
The train is there, you could also get Crossrail (old Heathrow Connect) to Hayes & Harlington (or Ealing Broadway). Heathrow Express into Paddington and then out again ends up being faster because the fastest trains are the limited-stop expresses, but that's a question of rail network optimization and happens in all sorts of places (e.g. back when I lived in North London the train to Cambridge went through the rail station nearest my house (Harringay), but it was quicker for me to go into King's Cross and out again). Physical distance has very little to do with it. Most people arriving at Heathrow are not going to Bath (a city of less than 100K, compared to London's 9M) and most people going to Bath are not coming from Heathrow; having to change at a hub isn't that unreasonable.
I wasn't really suggesting that the intercity rail system should be designed around my needs as an occasional intercontinental traveller to Bath.
However, the rail system exists. Every train stops at Reading. The trains also go to the capital and largest city in Wales (Cardiff) and Bristol (the 10th largest district by population in the UK). Access to this intercity rail line requires back-tracking into London, something that I don't consider absurd, but do consider a bit silly.
Also, taking population considerations and making them central can lead to strange results in the UK. 1/6th of the population lives in the London metro. While that's important from some perspectives, it can't be the overriding principle for integrated transportation.
I don't see why it shouldn't be? A more direct line to Reading has long been planned (indeed was being worked on until COVID-19 hit), and I imagine if passenger numbers recover then it will eventually get built. But good access to London has taken priority so far, and that seems right too.
The train is there, you could also get Crossrail (old Heathrow Connect) to Hayes & Harlington (or Ealing Broadway). Heathrow Express into Paddington and then out again ends up being faster because the fastest trains are the limited-stop expresses, but that's a question of rail network optimization and happens in all sorts of places (e.g. back when I lived in North London the train to Cambridge went through the rail station nearest my house (Harringay), but it was quicker for me to go into King's Cross and out again). Physical distance has very little to do with it. Most people arriving at Heathrow are not going to Bath (a city of less than 100K, compared to London's 9M) and most people going to Bath are not coming from Heathrow; having to change at a hub isn't that unreasonable.