You're a train

Scene: at the train table

D: I want to go on the bridge. 

C: No! I'm on the bridge!

D: OK. I'll go to the shed. Beep beep! Beep beep!

C: No! You're a train. No beep beep! You whistle. 

D: Oh. Chug-a-chug-a-chug-a peep peep!