Most Google searches and even the Glacier Express website says 'ski luggage' is not suitable for the train. However, Reddit forums are saying it's totally fine and you just need to arrive early. The theory is it's not a restricted item there's just limited space. Anyone feel confident one way or the other on this?
if Reddit says it is so it must be.
Me, I'd take it from the horse's mouth (and you checked) rather than miscellaneous anonymous people like me.
If you don't believe the GE website - maybe call or email or socials them?
Well, I have taken my boots as well as large luggage. But I generally rent skis in Europe. They are reasonable, and saves the lugging around airports, trains, and buses and non-ski destinations. Most airlines charge extra to haul skis as checked luggage, although Swiss says they will haul for free.
Nobody can tell you whether or not you'll get to carry your ski gear on a particular departure, space is limited and it will depend on who turns up on a given day and time. As I live here, I often take a trip on that train, sometimes there is very little luggage and other times a large group turns up with very big bags - Asian tourist groups usually have very large wheelie suitcases and there is no space available.
As we say here "This is Switzerland" (TIS) and we are a rule based society, so the staff will have no qualms about refusing you or requiring you to unload if the space is needed for other passengers normal luggage. So the question you need to answer is what is your plan B if you are refused boarding...