I'm on route 8 and want to go across Tubline bridge. I believe I have to..

The Tubline Bridge can be found east of Route 8. The Tubline Bridge isn't found anywhere near the Desert Resort.

Hope this helps!

I'll assume you've already done what you've had to do at Dragonspiral Tower.

The Desert Resort is found off Route 4, at the end of a spur that starts below the motorway flyover.

Once you get to the Desert Resort, go through the checkpoint and just keep trekking straight ahead from there (unless you want to venture off to gather some items) until you come to a set of ruins. This is the Relic Castle. Go in the entrance to the east.

Once you're inside, take note of the quicksand — if you try to run on any part of it, you'll be sucked in and end up on the floor below, but if you just walk slowly and stay away from the centre, you'll be fine. Continue along the first level to the stairs (and get the Cover Fossil or Plume Fossil from the Backpacker) and fall down the quicksand at the end of the corridor. From that point forward, just follow the Team Plasma Grunts until you get to Ghetsis. He'll tell you that the Light Stone isn't there, and then you'll be summoned to the Nacrene Museum to pick it up. After getting the Light Stone, you should be able to cross Tubeline Bridge.