Thumper is a quarry machine with two arms and caterpillar treads. He is used for loosening rocks.
Thumper was built by the Sodor Mining Company and brought to the quarry to help clear rock, but his arms accidentally dislodged an enormous boulder which chased Rusty, Skarloey and Rheneas down the line before nearly crushing Percy and running into the sheds at the mine. He often got out of control on mining sites and is now used for slate mining.
Although Thumper never spoke, he has been said to be laid back in his approach with a "cool dude" attitude, though he can come across as accident-prone.
Thumper is not based upon any known quarry machine - at any rate, a machine of his type would be extremely hazardous to those around due to the lack of a window at the front of the cab, making the operator unable to see.
In addition, assuming that a window would be technically where the face would be, such a machine would be impractical in real life as well due to the end of its arms not providing a sufficient amount of area to loosen the rock on the sides and below and above the machine, let alone be capable of making a proper hole through the cliff face and properly loosening or clearing rocks. The arms appear to be on a non-flexible joint with the siderods attached to the inner arms appearing to be quite thin, thus potentially leading to the machine being more capable of damaging itself more than loosening or clearing rocks, unless being lightly placed in front of the rock / cliff face to properly function, which thus also means that the rock may only be lightly loosened or cracked. Thumper's treads appear to be based off the treads of a Panzer IV tank, albeit with the treads mounted backwards with the drive sprocket in the rear.
Thumper is painted orange-red with his name painted on the sides of his bonnet in white. His roof is painted black.
- Thumper was never referred to by gender in Rusty and the Boulder, rather he is referred to as "it" instead.
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