Walter seems to be impressed by that idea, although he doesn't ask how it is that Peck can choose where he goes back to. At first I thought maybe that supported the idea that you can only jump back into yourself -- and then I remembered that Peck wasn't in the train car, until he suddenly was.
ANYWAY. Walter says he knows why Peck hasn't jumped back yet: "Because you don't know how to. You haven't been able to jump back any further than the train." He stands up and pulls out his hidden microphone, and unplugs it. Outside, Broyles gets static in his ear and yells at someone to get the signal back online.
In the lab, Walter, who is a geneticist who couldn't really understand Peck's formulae, tells him that he's approximating the time curvature with a seventh-order polynomial, when, for the distance Peck requires, it should be at least nine. "I've read you too," he says by way of explanation.
An agent tells Broyles that they can't get the signal back online, because Walter turned the radio off. Broyles gives Olivia one of those bowels-evacuating glares and then orders via the radio for a team to get up there, now.