Servus, the MAIN and very often mentioned on this forum bottlenecks are:
ALL in ESpana, except the clearly non-touristy routes=where you thus also will not want to go
ALL over the border to/fro FRance in TGV and €*-limited quota and sky-high supplmt to pay too
NIghtJet over long distance.
In ESpana its very hard to impossible to avoid these REServ. For those to/fro FR it very often is=use regional zuge,=takes longer, more changes. Old-hand InterRailers would know how to-but it seems the young from today do not seem to have the slightest clue as they have been grown up with these planners online only.
Depends on your route and how fixed your route is.
Few trains like Eurostar, TGV/AVE between France & Spain can be sold out several days if not weeks in advance as they have just limited seats for Railpassholders.