Arya was staring at the lines, trying to interpret them the way O-Ren seemed to be.

"Where would I have seen a train?" she said defensively; she'd gotten so used to knowing every corner and feature of the island, to being Arya Underfoot again, that suddenly being somewhere entirely strange and new again, being lost, rankled some.

So she didn't add that she wasn't sure what a train was, precisely. Something like cars, only you couldn't steer, she thought.