This bridge acts as the gateway to north-west Unova, allowing you to explore these areas after defeating the Elite Four . To get there, simply Fly to Opelucid City (which you'll find has now been cleaned up after Team Plasma's attack).
Then head out of the east exit and through Route 9 . After that, you'll be in Tubeline Bridge! Once you enter it, you'll notice Battle Subway trains moving underneath bridge. Run what appears to be south along the bridge until you reach a Battle Girl in the centre.
Talk with her and she'll hand you TM43 - Flame Charge . Niiiiice! No one else on the bridge is important, so continue until you reach the gate on the other side. Talk to the Scientist on the north side of the gate to receive EnergyPowder , then exit the gate into Route 8 .