After hours of work, Tally takes a break with David. Her hands are all raw and blistery, so it's about time.
David takes Tally to see a collapsed tunnel and all the railroad tracks buried under there. (Oh, how romantic.)
The boulders seem pretty solid, but Tally thinks about how a small shift would bring down all that weight on them—which sounds (to her) a lot like Dr. Cable and the city (crushing her).
David brought her here because he can tell that she's taking this seriously. Some other runaways just think about this like a fun adventure, like camping out for a while.
But for David, this place is home—he was born here, since his parents were runaways.
And so Tally thinks that she can't call Dr. Cable because that would destroy David's home. (It's one thing to bring your runaway friend home; it's a much worse thing to destroy the only home a guy's ever known.)