noun
choo-choo train; steam train
Children's word for a steam train, mimicking the sound of the whistle and the puffing of the engine. Often used in songs and picture books.
子供が「汽車ポッポが来た！」と叫んだ。
The child shouted, "The choo-choo train is here!"
この絵本には汽車ポッポの絵がたくさんある。
This picture book has many pictures of choo-choo trains.
汽車 is the standard word for a steam train or locomotive, while 汽車ポッポ is a childish, onomatopoeic variant.
電車 refers to an electric train, whereas 汽車ポッポ specifically evokes an old-fashioned steam train.
Compound of 汽車 (steam train) and the onomatopoeia ポッポ, imitating the chuffing sound of a steam locomotive.