Funny Adult Joke: A boy in the USA asks his parents how come big trains don't have little trains

A little boy in the USA is learning about the world and it's wonders.

He is curious about how things work, and one day he goes to his father and asks,

"Daddy, if big dogs can have little dogs, how come big trains don't have little trains?"

Never unprepared for this sort of question, his father says, "Go ask your mother."

The dutiful son finds his mother:

"Mommy, if big dogs can have little dogs, how come big trains don't have little trains?"

"Well," she smiles, "why don't you go on down to the train station and ask the conductor?"

So off goes the little boy to the local train station. He sees the conductor in uniform and asks:

"Excuse me sir, can you tell me why if big dogs can have little dogs, how come big trains don't have little trains?"

The conductor looks down at him,

"Son," he says, "here at Amtrak our trains always pull out on time."