[ignoramous]

For France, Spain, and Italy, only the inter-country and the high speed rails (TGV, Renfe, Iryo, Frecciarossa etc) require reservation?

In Switzerland, some of the touristy intra-country routes (ex: Tirano-Chur, Interlaken-Lucerne) may require (a rather expensive) reservation.

[phreeza]

For Switzerland, that's only true for the special tourist train panoramic carriages on the route, there are normal trains running on the same routes, too, that don't require reservations.

[andrewshadura]

In France, some destinations are served only by TGV. In Italy, taking local trains can take an eternity.

[riffraff]

but with interrail the "useful" thing was to travel overnight, because that saves you one night of $$ for loding, so slow is actually fine.

I don't think fast trains (TGV, Frecciarossa etc) were ever covered by the interrail ticket anyway, were they?

[andrewshadura]

They are covered, but you need a surcharge or a seat reservation, or both.