in italy we very rarely paid a supplement, but there were a couple of time when we were on night trains when we had to pay a supplement for a couchette. the on train i remember this happening on was from nice to venice, but it wasnt too expensive, only about 8 euros, so still cheaper than a hostel, and it meant we didnt waste our days travelling.
in france, we were trying to get from belgium to paris and then paris to monte carlo, which was surprisingly difficult! it would have been much easier to get TGV trains, but we were quoted 60 euros per leg of the journey, which we just didnt have, so instead we ended up taking it slowly n going via toulouse.
italy was much better!