On her trip, Tally sees what the railroads are all about—that's how the Rusties used to transport all their rubber ducks.
The next part of Shay's note says "Cold is the sea and watch for breaks" (5), which Tally ignores a bit. Which is why she almost falls down into the sea when she crosses a crumbling bridge around daybreak—because she wasn't watching for breaks in the railroad tracks.
So Tally celebrates her near-death experience by making some of her dehydrated food marked "SpagBol." It turns out to be Spaghetti Bolognese and very good after a night of hard hoverboarding.
The fourth part of Shay's note says "At the second make the worst mistake" (31), which doesn't sound fun at all. So at the next crumbling bridge, she… goes to sleep and lets the solar-powered hoverboard charge up. (It flips open several times in order to show more panels and charge faster.)
But before she goes to sleep, Tally looks through her bag and discovers that all Dr. Cable gave her was SpagBol. Which is kind of a jerky move: sure, get someone to betray her friends, that's not totally cool. Sticking her with one type of food for a week--that's just mean.