- “We should have left this part of the island alone...”
- ―Sir Topham Hatt
High up in the mountains, there is a mysterious boulder that has stood alone for a long time. But one day, a new quarry is built near the mines on the land below and Rusty is afraid the giant boulder will fall onto the line. He confesses his fears to Thomas and Percy. Edward arrives with a new machine called Thumper to help with the mining process. Soon, Thumper is working hard. The men are pleased, but no one bothers to check the boulder. However, it starts raining and the workmen leave. Rusty gazes up and shivers at the boulder above. Suddenly, a large slab of rock lands on the tracks. Rusty is shocked and worried by this, but his driver is concerned. His driver thinks the rock has been loosened by the rain, but Rusty believes the boulder is telling them to go away. As they leave, a face mysteriously appears on Boulder and then disappears.
The next day, Thumper begins working harder than ever before. Suddenly, Boulder begins moving and Rusty is back at the same location from yesterday. He tell his driver that Boulder is about to move, but he does not believe him until Boulder falls down and lands on the tracks in front of them and begins to chase Rusty. Boulder rolls along the tracks as Rusty quickly drives himself backwards to get away from it. Rusty and his driver stop at a junction and wait for Boulder to pass by. Then, they soon realise that Boulder is suddenly behind them. Just ahead, they see a small junction that goes uphill, so Rusty takes the track going uphill while the Boulder thunders by past them.
Meanwhile, Skarloey is on his way to the quarry, but he sees the Boulder heading for him and immediately reverses. He soon slips into a siding, while the Boulder passes by, destroying the water tower in between the tracks. Skarloey tells Rusty that they have to warn the yards, but Rusty does not know how. Boulder rounds a bend and charges towards Rheneas, who races back as fast as he can to get away from Boulder until he crashes into a siding with the Boulder just passing by him, much to the relief of his driver. At the yards, Percy is collecting trucks when he hears and sees Boulder approaching. It rolls past, narrowly missing Percy and crashes into the neighbouring sheds, destroying them, and engulfing them on fire.
When the Fat Controller inspects the damage, he decides to close the mine. Then, looking at Boulder amidst the destruction it had caused, he lamentably concludes that they should have left this part of the island alone. They move Boulder to a small hill close by the yards so it will never harm anyone or anything on the island anymore. Rusty is sure that, on a clear night, it is gazing high up at the mountains in the distance and that its sighs are being carried on to the wind where it once used to stand, proud and silent.
- Sir Topham Hatt
- Thumper (does not speak)
- Boulder (does not speak)
- Terence (cameo)
- George (cameo)
- Butch (cameo)
- Going by production order, this is the twenty-sixth episode of the fifth series and the series finale of the fifth series.
- Boulder's face is based upon one of Gordon's face masks.
- The same model for Boulder is used for both the large-scale and normal-scale sets.
- This episode marks the only appearance of Thumper to date.
- This is the only episode where Terence appears without his driver.
- In this episode, the quarry was brand new and closed in a short amount of time, meaning the events of the fifth series episodes, Bye George! and Duncan Gets Spooked must have taken place during the events of this episode, as they also took place at the Boulder Quarry yards at the base of the mountain. Furthermore, George is seen at Thumper's dig site before Boulder gets loose, meaning that he served his week-long punishment and was brought back to the quarry to help.
- The scenes where Boulder chases after Rusty, Skarloey and Rheneas are references to the boulder chasing scene in the Indiana Jones film, Raiders of the Lost Ark. This reference was later used again in the Thomas' YouTube World Tour short, Thomas Goes for Gold, and the twenty-third series episode, Mines of Mystery.
- Rusty's small-scale model is used at the beginning and end of the episode, whilst his large-scale model is used for the remainder of this episode.
- Rheneas' small-scale model was used when he was crossing the viaduct, whilst his large-scale model is used for when the boulder starts rolling towards him and he crashes.
- A small connector is attached to Rheneas when the camera gets close-ups of him, while the boulder chases after him.
- This is one of Davey Moore's favourite episodes alongside the first series episode, Thomas' Train and the seventeenth series episode, Luke's New Friend.
- In the US version, the sound of Boulder thudding on the ground has a much loud noise than in all the other dubs in this episode.
- Some of the music throughout the runaway is heard in the US version.
- In the US version, music can be heard when Edward introduces Rusty to Thumper.
- In the UK version, the line "EYYOIIIKES!!!" was re-used from the fifth series episode, Busy Going Backwards for Skarloey and Rheneas, when Boulder charges after them.
- In the US version, the sound of the Boulder destroying the water tower besides Skarloey sounds quieter than in all the International versions in this episode.
- In the US version, the crash sound was heard when Rheneas crashes into a siding with Boulder just passing by him.
- Butch gains a face in this episode, as in his other appearances besides Horrid Lorry, he faced away from the camera.
- This episode marks a few things for Rheneas:
- Rheneas' only speaking role in the fifth season in the US version.
- His last speaking role and appearance until the sixth season episode Dunkin Duncan.
- This episode is the last time that Percy’s original wincing face was used on-screen.
- Due to his model being changed, Rusty's face constantly changes size throughout the episode.
- Boulder is still covered in moss while it chases Rusty. However, once it appears behind him, all the moss has vanished.
- In the close-ups of Rheneas running backwards from Boulder, a camera rod can be seen attracted to his coupling hook.
- In the close-up of Rheneas' driver, his right (viewer's left) pupil appears to be sloppily painted. Also, he appears to be in Rusty's cab instead of Rheneas'.
- In the US narration, when Percy is at the quarry before Boulder arrives, the British term "trucks" is used.
- In the second close-up of Percy when Boulder is approaching, it appears that the shed has already been destroyed as the wood from the shed is laying on Percy's trucks.
- Percy: Where's all this rock coming from?
- Rusty: The new quarry. This mountain rock is good for many things, although it's dangerous up there.
- Thomas: Why?
- Rusty: Because of a big boulder. I think it's watching me.
- Percy: How can it be? Boulders don't have eyes!
- Rusty: That's as may be, Percy, but there's something strange about this one. (US: "Well, Percy, there's something strange about this one.")
[Edward arrives with a machine named Thumper for the quarry]
- Rusty: What's that?
- Edward: It's called Thumper. Apparently, it helps collect the rock faster.
- Rusty: [to his driver] I think it's the boulder wanting us to go away.
- (Skarloey makes his way up to the quarry with some trucks/cars, but encounters Boulder)
- Skarloey: (reverses to safety) US: HELP! / UK: EYYOIIIKES!!!
- (Boulder catches up fast, destroying a water tower in the process but Skarloey safely reaches a siding)
- Skarloey: We must warn the yards!
- Rusty: Yes, but how?
- Rheneas: It's running loose! US: Hurry! / UK: EYYOIIIKES!!!
- [Rheneas races in reverse as the Boulder chases him, and then manages to safely race into a siding]
- Rheneas' Driver: (relieved) Rather a smash than a squash! (US: "Better a smash than a squash!")
- Wooden Railway - Boulder Mountain Set & Boulder Expansion Pack
- Tomica (discontinued)
- Take Along (discontinued)
- TrackMaster (discontinued)
- Wind-up Trains (Japan only) - Boulder
- Motor Road and Rail (Japan only)
In Other Languages