Hi all,
Just reporting back from the Vallée des Merveilles!
Firstly, a notice: the guide informed us that from the end of the summer (ie this September 2024) the line to Tende will be closed completely for
16 months for modernisation until December 2025. So if you want to visit, do so this summer! (Or prepare for buses!)
Another source here (in French):
Les grands travaux de réfection et d'amélioration de la ligne entre Nice, Breil-sur-Roya et Tende vont entrainer l'arrêt total du trafic des trains à partir de septembre 2024. Un choix pour mener des travaux rapides mais qui vont occasionner des soucis d'organisation pour les usagers.
www.francebleu.fr
On the bright side they are hoping it will mean the speed limit can be brought back up to 80kph or even 100kph. Currently the train is slower - we were told around 40kph - particularly once you are in the valleys.
Sadly we had no time to visit Tende, although we did ride the train all the way up and back, stopping off at Breil-sur-Roya on the return. The tour guide on the way up was excellent, very knowledgeable guide who gave both French and English commentary all the way up to Tende. Not sure about other languages but she came through the train before we left Nice to let everyone know which train to be in for the narration (and thereby to be in the other one if you can't stand it!) and to see who was on board.
There was a mechanical failure on the train and we were expecting a delay of some sort but it ended up only 5 minutes late which we made back in no time. The driver even had time to slow down at some of the more impressive landmarks!
There are a lot of trees which means if you want pictures you will need to be ready to take them in clearings! Mobile signal was surprisingly even working in Tende(!) although a lot of the line is tunnel as well so keep that in mind. In any case we didn't use our phones for much other than photography

There is definitely a better side to be on! (As I recall it is the left side as you go up) The train empties out further along the valley (many people seem to get off at or before La Trinite as well) so it's not the end of the world, but I can imagine in the summer you might be a bit disappointed.
Breil-sur-Roya was a lovely village (We visited on a Sunday so there was only the one restaurant open and not much else) and well worth the time we had there. Indeed not very touristy but more the "old France" that we were looking for.
Sospeil seemed popular with cyclists, although I can't comment on that

Unfortunately in October 2020 the Roya valley flooded catastrophically along with a lot of the valleys in Alpes-Maritimes, and there are still reminders of this as well as some construction about and some more modern road bridges down in the valley replacing the ones which were destroyed. Whether construction spoils the view is up to you - there is still plenty historic to see and they seem perfectly aware of this heritage.
Well worth a visit if you are spare a few hours in Nice (and certainly once the third train to Tende starts up later in April)!
Edit: There is also a railway museum in Breil, run by a local railway workers group AIUI. They had quite a few trains out in the station area as well as the sheds, including the previous Train des Merveilles diesel railcars. Not sure if it is open yet but I imagine it will be open from May if not - check their Facebook page.