One Friday, little Tommy said to his mother, "Mummy, you know how big cats have little cats and big dogs have little dogs - why don't big trains have little trains?"
"I don't know," She said, then cunningly added, "Why don't you ask Daddy when he gets home?"

So when his father came home after work little Tommy said. "Daddy, you know how big cats have little cats and big dogs have little dogs - why don't big trains have little trains?"
Daddy squirmed at how to answer the rather embarrassing question, then had a brainwave. "Tomorrow's Saturday. Why don't you go down to the station and ask the engine driver."

So next day Tommy walked to the station, waited for a train, and then went up to the driver who was standing on the footplate. "Please, Mr Driver, I have a question."
"Fire away" said the kindly driver, "and I'll see if I can answer."

"Well, you know how big cats have little cats and big dogs have little dogs - why don't big trains have little trains?"

The driver laughed, patted Tommy's head and told him. "It's because this engine I'm driving pulls the mail train - and the mail train always pulls out on time."