Leave the community center and go back to the dam. Move the propeller so that the left side is lower than the right and throw the switch again, closing the path.
Return to the beam emitter locomotive. Throw the junctions until you've backed up the cart against rocks, between the community center and the town.
Without getting out of the cart, throw the railroad switch, and continue on.
Continue along the track until you get to a sheet metal fence. Get out and open the gate to this area.
Get back in the cart. Now point your laser through the gate. Try to hit the rocks blocking the way through the passage. Go through.
You'll find yourself in what looks like a junkyard. Move down the right path. If you want, you can head upstairs and look at the manifest. When you're done, head back down and open the sliding door that the tracks lead under.
Now, move forward and take the right fork at the tracks. Exit through the purple portal.
Once back, head through the door, and take the purple portal here. Go down the rails until you get to another purple portal. Now, you'll find yourself by Farley's Community Center.
Go through the Community Center, then hang right. Go up the stone stairs, and walk through the purple portal here.
Once through, hang left to cross over the top of the dam wall, which is now a walkway.
Flip the lever to drop the stairs.