I do wonder if this is showing the flaw with Japenese high speed train technology when applied to Europe, their inability to cope with the transition between high speed lines and conventional lines? Unlike most of the rest of the world Japan developed their high speed network as totally segregated, with a big factor being the use of standard gauge wheras their normal network is narrow gauge. When we developed the HSTs, as well as running on conventional but good quality lines at 125mph, they also had to be designed to cope with running on old jointed track with wooden sleepers on secondary lines, often still at pretty good speeds. Whilst the secondary lines have improved, there are still sections of the HST network that involve running on old tracks. The TGVs too were developed both to run at very high speed on dedicated lines, but also to run at high speed on good conventional lines and to run onto much poorer quality secondary routes, in fact the investment in dedicated high speed lines in France did lead to the rest of the network being pretty neglected, and only fairly limited sections have been brought up to UK "high speed" standards to allow 200kph running by TGVs, basically the parts of the main TGV network that are on classic lines (such as the approaches to major cities). TGV routes off the main network still involve much running on jointed track with wooden sleepers under semaphore signals.
As well as the issue with track quality between different types of lines there is also the issue of cornering. High speed lines are pretty straight wheras conventional lines have sharp curves. The same issue effects light rail where the ability to take very sharp corners for street running does have a negative impact on running at higher speeds (DLR seems about the worst for that, the fast sections can be very interesting). As well as bogie design and yaw damper tuning, wheel profile also plays a role.
