Up to TGV, future high speed rail had idolised the Shinkansen in Japan, always looking at completely separate alignments throughout, not necessarily noticing that the reason for that in Japan was that they were a different gauge. TGV brought the idea of high speed alignments on the cheap to build rural bits (completely lost on HS2 it seems), and running on to existing alignments with all the upsides of incremental development that provides as well.
At least Shinkansen was on rails, where politicians and high-placed amateurs are often fixated by hovertrains, monorails, maglev, hyperloops, and anything else even more incompatible and unworkable.
APT was a development because the West Coast is a far more corkscrew alignment than out of either Kings Cross or Paddington, and there's hardly any of it suitable for over 100mph without tilt. It was a shame they did not do more development of the prototype before rushing to the pre-production units, which had a notable number of fundamental points which the Pendolino etc shows were just wrong.