Here's how Boulder mountain goes in Return of Boulder.
Narrator: The next day as the engines were working, the Crusaders were closely watching Rusty.
Sweetie Belle: Okay, Rusty. Let's see what's really going on.
[They follow behind him]
Narrator: Later in the afternoon, the Crusaders notice Rusty taking a different track.
Scootaloo: Huh? Why's Rusty going that way?
[They follow him down the other lane]
Apple Bloom: This is very strange. Rusty almost never takes a different track.
Sweetie Belle: Where is he going?
Narrator: The Crusaders continue to follow Rusty until they come to a a small mountain. And the Crusaders then hide in a bush and watch, and they are shocked to see Boulder on the top of the mountain!
Narrator: As the Crusaders watch, the wokemen who were with Rusty build pikes around Boulder so he won't fall.
Scootaloo: So that's what's going on.
Sweetie Belle: He was looking at Boulder!
Apple Bloom: No wonder he was actin' all funny-like.
Scootaloo: Yeah, I would be too.
Sweetie Belle: We should tell the others.
Apple Bloom: Ah' don't know about doin' that. What if Rusty finds out we did?
Scootaloo: Don't worry, he doesn't know we're here.
Sweetie Belle: Well, let's go before he notices us.
[the crusaders then sneak away inside the bush]
Rusty: [hears the rustling] Huh? [he looks back and the bush is already gone] Hmm, I thought I heard something.
Workmen: Rusty! C'mon!
Rusty: Oh! Coming!
Narrator: Then with determination, the CMC fly back to Berk and soon are explaining everything.
Scootaloo: And that's how it happened!
Snotlout: So, let me get this straight. You're saying that Rusty is distracted with.... a rock?
Apple Bloom: Eeyup.
Snotlout: That's stupid! What's the big deal? It's just a rock!
Sweetie Belle: No, Snotlout, you don't understand. Rusty's had a crazy experience with that Boulder.
Snotlout: What are you talking about?
Astrid: Yeah, how can a giant boulder have such a huge impact on a narrow gauge diesel?
Apple Bloom: Well, long ago, that boulder used ta' be on a realla' tall cliff...
Narrator: The next day the sun shown, Thumper was wroking harder than ever. Suddenly, Rusty noticed something.
Rusty: Boulder's moving!
[we see Boulder slightly move]
Rusty's Drvier: Don't be so daft, it can't!
Narrator: Said the driver. But it could!
[Then Boulder fall off the mountain and lands on the track! Then he starts rolling!]
Rusty: It's rolling along our line!
[Rusty then reverses back as the boulder chases him, and then he stops by a tree]
Rusty's Driver: We'll stop here until Boulder passes by.
Narrator: Said the driver. But Boulder was no where to be seen. Then...
Rusty's Driver: (looks back) Oh, no! It's behind us!
[Rusty then takes off as the Boulder bumps the tree and chases him]
Narrator: Just ahead, they saw a small junction. One line went uphill. Boulder thundered past.
[Rusty drives on the upper track as Boulder races down the line, then we now view Skarloey]
Narrator: Meanwhile, Skarloey was making his way to the quarry. Then he saw Boulder!
[we then see Boulder racing for Skarloey]
Skarloey: [reverses back] HELP!!
Narrator: Boulder was catching up fast!
[Skarleoy then drives into a siding as Boulder races past and knocks over a water tower]
Skarloey: We must warn the yards!
Narraor: Shouted, Skarloey.
Rusty: Yes! But how?!
Narrator: Called, Rusty.
[Boulder contiue to race down the line, as we see Rheneas]
Narrator: Boulder rounded a bend, there, ahead was Rheneas.
Rheneas: It's running loose!
Narrator: Yelled Rheneas.
[boulder then appears in front of him and rheneas races back]
Narrator: His driver drove him back as fast as he could!
[we now watch as Rheneas races back with Boulder hot on his tail, then they race across the viaduct, and then Rheneas speeds into a siding and smashes into the buffers while Boulder races past and down the line.]
Rheneas' Driver: Better a smash than a squash.
Narrator: Sighed his driver. At the yards Percy was collecting trucks, then he heard boulder!
[we see Boulder approaching from around the corner]
Percy: [whistles} Oh no! It's heading straight for me!
Narrator: Squeked Percy.
[Percy shuts his eyes as Boulder passes by and then it destroys a shed in a firey inferno!]
Narrator: When Sir Topham Hatt expected the damage, he decided to close the mine. Then he looked at Boulder.
Sir Topham Hatt: We should had left this part of the island... alone.
Narrator: They moved Boulder to a hill close by the yards. Rusty was sure, that on a clear night, it is gazing up on the mountain, and that it's sighs are being carried on the wind, till where it once used to stand, proud, and silent. I wonder if Rusty's right, don't you?
Snoutlout: Ah, that makes sense.
Astrid: Oh man, I'd feel the same way if something like that happened to me.
Fishlegs: Wait, you don't think that the Boulder, could possible roll off by itself again?
Scootaloo: Maybe, we don't know.