Here is information on some of the scenic routes in Switzerland. On all routes it is possible to travel without reservation if you avoid the panoramic trains/waggons and use the regional trains or the normal waggons that are connected to some of the panoramic trains.

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

https://www.vigezzinacentovalli.com/en/info-servizi.html

Its about reverse; but its more a case of picking the right normal trains and NOT the expensive and praise too much to over-excited tourists special panorama cars (its mostly in the swiss). Then its ´free´ or no extra to pay.

There used to be country sections in main site also mentioning what in that country would be thought of as being scenic lines, but there was a recent reshuffle and not that easy to find.

In a free newsppr I got last week which reprinted some CNN (or whatever) article about 10 ´most scenic´ rides in the world there were 4 in EUR, from what I recall:

Swiss: bernina to ITaly, St Moritz-Tirano

FRance: little yellow metro de Pyrenees (from Perpignan west into mountains)-I never ever see this mentioned here, whereas other german sites swear by it and go out of their way to ride it

GB/Scot: toward Fort William an the western Hebrides.

NOrway: Flamsbanan, just east of Bergen- ´NO in a nutshell´ (only discount with pass)