An extension cord- There seem to be only 3 outlets on the train and they are not placed in areas where you could sit and work. Also, one with multiple ends will allow you to make friends by sharing the power.
Food and water- the food on board is neither good nor cheap. Get some food and drink youll like, and maybe some extra to share if you want to make friends.
A blanket- I was ok in the summer, but in the winter youd probably want something to curl up under.
Eyemask & ear plugs- My earbuds kept things quiet but it would have been nice to shield my eyes from the interior lights.
Something to read- When your electronics run out of power you’ll need something to do while delayed 5 hours on a mountian pass. I was able to borrow a book on the great speaches of the 20th century.
All in all, this was an impressive trip and n interesting, albeit slow, way to see the country.