Max says the best way to get from Williamsburg to Riverdale is to take the J train to the 4 train, then change at Grand Central for a train to Riverdale. There are no subway trains from Grand Central that go to Riverdale - from Grand Central, they would have to take the shuttle or the 7 train to Times Square and change for the 1 to Riverdale. There is, however, a commuter train from Grand Central that stops in Riverdale, but that would create a continuity error, as the girls are presumably too poor to opt for a more expensive, but possibly shorter, train ride, rather than taking a convoluted subway route with multiple, but free, transfers.