Crossword Genius

Half opting twice for train (4-4)

Ross

I believe the answer is:

choo-choo

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'train' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for choo-choo that I've seen before include "Child's railway train" , "Baby train?" , "(Childish) train" , "A baby's train" , "A child's train, sounds repetitive" .)

I've seen this clue in The Telegraph.
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