Rollercoaster Level in Super Mario Galaxy (Wii), Not New Super Mario Bros. Wii

You're thinking of World 8-4: "Rollercoaster Galaxy" in Super Mario Galaxy (2007), not New Super Mario Bros. Wii (which has no rollercoaster stage).

Unlocking World 8-4 in Super Mario Galaxy

1. Collect 120 Power Stars (complete all main stars in Worlds 1–7).
2. The final dome (World 8) unlocks on the Planet Select screen.
3. Enter World 8 → beat 8-1, 8-2, 8-3.
4. 8-4 "Rollercoaster Galaxy" appears.

It's a fast-paced rail-riding level on a mine-cart star; steer with Wii Remote tilt.