Why did he suddenly talk about "boiler"?
Native speaker’s answer
Rebecca
He talks about a "boiler" because he is a steam-powered train, and this type of train has a "boiler". A "boiler" on a steam train is an area of the train where water is boiled to create steam, and that steam is used to power the train. His statement "I make my boiler bubble" here means "I make myself laugh".