So, we've left the city: seems like a good time to start a new section. Welcome to Part 2, "The Smoke."
Tally sneaks out one last time—this time on official business (for the secret police—whee?).
She's got a fancy new hoverboard that can go quite fast. Which is good because she needs to get to the Smoke real fast or they might realize that she left after her birthday, which would be weird.
So Tally jets off to the Rusty Ruins to deal with Shay's weird note.
The first part says to "Take the coaster straight past the gap" (18), which must be a reference to the roller coaster.
The second part says "Until you find one that's long and flat" (25), which Tally eventually figures out is a reference to a long, straight roller coaster-like track—a railroad track. (But of course, Tally doesn't know about railroads. Also: bellbottoms. They're a complete mystery to her.)
The third part of the note says to stick near the sea, so that's the way Tally rides. This sure seems easy to Tally—which probably means that things are going to get hard soon.