Kind of dog — mine doesn't reach railway (6)

I believe the answer is:

collie

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'kind of dog' is the definition.

I don't understand the rest of the clue.

(Other definitions for collie that I've seen before include "Popular sheepdog like Lassie" , "Sheep-herding dog" , "Sheepdog with long pointed muzzle" , "Type of sheepdog" , "Long-haired sheepdog" .)

I've seen this clue in The Guardian.

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