They take the tickets to the train station, where yet more plans are made. [...] The plan itself is a sharp reminder of the one difference that still remains between Manny and Meche: she has a ticket on the train. He doesn't.

Actually, there never is any indication that the plans were made at the train station for Meche alone to bring the tickets back. If so, then this answers your earlier query about Meche's accompanying Manny to Nuevo Marrow; it may have been decided before setting off that only she needs to take the trip back.