also: しゅっしゅっぽっぽっ
adverb taking the 'to' particle
chuff-chuff; choo-choo; chug-chug; puff-puff
Onomatopoeia for the sound of a steam engine, often used in children's contexts or to evoke old trains.
See also: ぽっぽ
The old steam locomotive started off with a chuff-chuff-puff-puff.