The phrase "chugga chugga choo choo" is often used to imitate the sound of a train starting and moving along the tracks. In this common onomatopoeic expression:

• "Chugga" represents the rhythmic puffing or chugging sound of the train's engine.
• "Choo choo" represents the whistle sound of the train.

Typically, there are two "chuggas" before the "choo choo." So the sequence goes: "Chugga chugga choo choo" This mimics the train's engine chugging twice before the whistle blows. If you have a specific song, rhyme, or context in mind, let me know!