Norway In a Nutshell

I don't recall food availability on the train from Oslo to Flam, but there is food available in Flam and on the boat cruise. I would pack some snacks and beverages just in case. It takes 6 hours to get to Flam from Oslo. A great trip, but a long day.

As I recall there was snack type food on the ferry and I believe there is a cafe car on the high speed train from Myrdal to Oslo. You may have time to get something to eat in Flam depending on your train time. I'd suggest eating a big breakfast and packing a lunch/snacks.

There is a snack bar on the ferries (hot and cold food and beer). With the exception of the Flamsbana (Myrdal-Flam), there are snack bars on the trains.

Bergin Line Rail food service:
https://www.vy.no/en/travelling-with-us/train/travel-info/cafe

Menu:
https://vy.meny.togservice.no/en/

Bakery and Cafe near Flåmsbana termination on the way to the ferry terminal.

We just did this trip from Bergen to Oslo with the ferry. At the ferry terminal the one restaurant there told us no food was available on the 2 hour ferry, so we quickly bought stuff and wolfed it down, as we were also told no food could be carried onto the ferry. So we were surprised to board the ferry to see a very nice selection of sandwiches and salads as well as sweets. We reported this to the ferry cafe people who rolled their eyes. Apparently that restaurant has done this before. The Flam to Oslo leg has a cafe car on the Myrdal to Oslo leg which offered a decent enough selection of food and beverages.

The little cafe at the Mydal RR had decent pre-made sandwiches on focaccia or some similar bread. (Norwegian bread is tasty.) The cost was just under 8 NOK for a good-sized sandwich.

The bakery in Flam near the Gudvangen ferry dock had a nice smoked-salmon sandwich ready to grab during breakfast hours--again just under 8 NOK for a sandwich of generous size. Cheese and ham-and-cheese were other options, as well as various rolls. There is signage prohibiting outside food and drink at the ferry dock.

Flam has a little supermarket that's a bit farther from the dock. If you have time to get there, you can pick up packages of sliced cheese, fruit, etc. If you zoom in on Google maps, I think you'll be able to identify the supermarket easily. As I recall, it's a Coop. I've found Wasa crisp bread indestructible; I keep a package on hand for consumption with cheese when the opportunity presents itself.

In many cases the schedule doesn't allow time to grab anything at Gudvangen, even if there is a food source there; the bus is often waiting at the dock when the ferry arrives.

I didn't go looking for food at Voss, but the RR station is in a commercial area, so I believe options should be available, quite possibly including a small supermarket.

Note that most supermarkets in Norway are closed on Sunday. Larger cities like Oslo and Bergen will have convenience stores (like 7-11) open that day, but I think in smaller towns along the Nutshell route you'd probably be limited to whatever cafes and restaurants choose to be open.

I had the same confusion after reading RS's book for that section. I was not clear whether I could drive or I have to rely on the public transportation and how to break the journey on my own as I see the 1-day or 2- day trip seemed very intense to me.
I found this website very helpful: https://www.heartmybackpack.com/norway/nutshell-tour-diy-review/
I even bought her guide book for sourthern Norway.