French TGV runs on a significative amount of non-highspeed track though, and it seems to work good enough (about the same speed as China's HSR, and it has been working for decades).
I understand the TGV uses weight limitation and careful scheduling to mitigate the cost of other traffic (both wear-and-tear and scheduling), and as in the case of China also has sections of line that are unavoidably mixed, but used, as cost effective alternatives are not attractive. And I think that's the crux: a trade-off between cost and benefit. A new high-speed line is still margins cheaper than a maglev, though that's not to say I'd not like to see maglevs.
