Green Star #1
The Bullet Bill Express wastes no time presenting you with collectibles. You have to quickly board the train as soon as you regain control of your character, and reach the first Green Star before it’s scrolled off-screen. Be mindful of the Bill Blasters protecting the star.
Back on the ground level, the second trio of ? Blocks contains a Cannon Box.
The second car introduces Flopters, little floating baddies that descend toward you. Meanwhile, in the distance, Cat Bullet Bills are being fired by another train running alongside of this one. These Cat Bullet Bills will home in toward you, so be ready to react to them.
Green Star #2
The second Green Star is found atop a Goomba Tower on the third train car. The Cannon Box makes retrieval easy, but if you’ve lost it, you can use the Cat Bullet Bills as a springboard, or you can let the Cat Bullet Bills take out the Goomba Tower for you.
It is possible to find a Mega Mushroom on this car. This makes the following sections a lot easier while the Power-Up lasts.
Green Star #3
Eventually you encounter a Bill Blaster firing massive Cat Banzai Bills at your train, and shortly after that you find a pipe that will take you to the other train. On that train, Captain Toad is locked up inside a wooden crate. Saving him earns you the last Green Star. Take the Warp Box back over to the first train.
Stamp
The front of the first train is protected by two Fire Bros. Don’t take the pipe they are guarding right away! Instead, drop down to where the second Fire Bro starts off and get past him to retrieve the Stamp.
Boss Battle: Pom Pom, Round One!
Pom Pom fights completely differently than her brother. She teleports around the arena, leaving a trail of smoke in her wake that eventually splits up. Where these trails end up is where a Pom Pom will appear. Only one of these Pom Poms is real, though. Look for the Pom Pom with the bright pink ninja star—that’s the one you want to attack. Once the Pom Poms throw their ninja stars, you have very little time to catch the real Pom Pom before she uses her magic to teleport again.
Every time you stomp on Pom Pom, she produces more shadow clones with each attack cycle until you finally hit her with three stomps (or lots and lots of fireballs, if you’re equipped for it). After you take her down, a Warp Box appears to take you to the Goal Pole zone.