[roelschroeven]

I'm not sure that's entirely correct.

If https://en.wikipedia.org/wiki/High-speed_rail_in_France is to be believed, the high speed lines for TGV are primarily but not exclusively used for TGV:

"LGVs are reserved primarily for TGVs. One reason for this is that line capacity is sharply reduced when trains of differing speeds are mixed, ..." "Some stretches of less-used LGV are routinely mixed-traffic, such as the Tours branch ..." "Outside France, LGV-type lines often carry non-TGV intercity traffic, often as a requirement of the initial funding commitments."

I'm also not so sure about the quality of the rail bed:

"Track alignment is more precise than on normal railway lines, and ballast is in a deeper-than-normal profile, resulting in increased load-bearing capacity and track stability. "