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Doing some more exploring now that we have Rock Smash. This is the Underground Railnet, accessed from the west side. That means going through the place under the bridge where the hobo was camping out, and where notm got a Pansear.
Yes, through. Turns out the right side's an exit even though there's no indication an exit is there. This game has this problem a lot, like in the pictures of Shelly's bedroom where one side wall is actually an exit.