With such a long train ride, I would just want to get to Wengen as soon as possible. Getting off the train with luggage for such a short time seems like a hassle to me.
I’d pick up some pastries and sandwiches at one of the bakeries or the grocery store at the Salzburg train station. Note, one of the bakeries takes cash only.
My sister and I were in Salzburg this past Sept, and if you can believe it, our favorite restaurant was the burrito factory. We wandered in there one day for lunch and ended up returning a second time. The best tacos we have ever had. We are still talking about them. Stop in if you get a chance.
https://burritofactory.at/