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Train
–
train
endlessly,
train
endlessly
(4-4)
Train – train endlessly, train endlessly (4-4)
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The definition is at the beginning of the clue.
How the clue works
The definition is Train — specifically the childish word for one. Each "train endlessly" takes SCHOOL (to train or educate), chops off both ends (the S and the L), leaving CHOO, and since the wordplay does this twice you get CHOO-CHOO.