"Choo choo train starting up" is the phrase used to describe the sound of a small train starting. The phrase is a onomatopoeic representation of the sound the train makes when it starts.

The phrase is often used to describe the sound of a toy train or a small train in children's stories or movies.

Examples:

The pttle choo choo train went chugging down the tracks.
The children cheered as the choo choo train started up with its famipar "choo choo" sound.

中文翻译:

小火车开始缓缓前行了.
当小火车发出“呜呜呜”的声音开始行驶时，孩子们都欢呼起来。