Pokemon X and Y Walkthrough: Route 13
Sina and Dexio meet with you in the gate between Coumarine City and Route 13. They give you the Mountain Kalos Pokedex. The Punk Guy in the gate gives away Black Sludge, which restores 1/16 of a Poison Pokemon's total health each turn. If a Pokemon that isn't Poison-type holds it, it takes 1/16 of its health in damage instead.
Once you enter Route 13, be aware of the Pokemon burrowing underground. A battle will start if they reach you. All of the power plant doors are currently locked, so don't worry about them for now. Ride the rail to the east to pick up a Smooth Rock. It makes Sandstorm's weather effect last three additional turns. A Flame Plate can be found in the southeast corner of the route.
Head to the northwest corner of the route and ride the rail. As long as you have a Pokemon that knows Rock Smash, you'll be able to pick up a Burn Heal and TM 57 Charge Beam. On top of dealing damage, Charge Beam has a high chance of lowering the target's Special Attack one stage. The rail that loops around in the southwest corner of the area leads to a Sun Stone.
As long as you have the Plant Badge from Ramos in Coumarine City, the Team Flare member guarding one of the Power Plant doors will battle you.
Team Flare Grunt
The grunt gets out of the way after being beaten. Now the only thing preventing you from entering the power plant is a key. Fortunately, the grunt gives a not-so-subtle hint to its location. It's located in the big boulder just before the bridge. Use the Dowsing Machine if you're having trouble finding it. Take the key and open the door to the plant that the grunt was guarding.