The Tubeline Bridge connects Route 8 to Route 9. The surface of the bridge is a kind of metallic mesh, beneath which you can see some subway trains running.
Tubeline Bridge leads to Route 9. There are no trainers here unless it's night time on Friday. On the bridge, talk to the Battle Girl to get TM43 Flame Charge. Near the end of the bridge, the Shadow Triad will surround you and take you to Ghetsis. He will talk to you for a while.
Trainers on Tubeline Bridge
If it's night time on Friday, you will find Biker Morgann and Biker Jeremy here. Morgann has an L37 Bouffalant, and Jeremy has an L35 Joltik, L35 Scraggy, and L37 Krokorok.
After you defeat Jeremy, he will rename his gang after your lead pokémon.