"Why do the wheels stay on the rails?"

There was a small boy on the train. He was one of those delightful children who ask endless questions - and really want to know the answers.
I suspect he was about four, five at the most. Before we left the city station he had a good long look at my tricycle. Almost before I had it in the right position in the proper place he was out of his seat next to the bicycle space to have a look at it. "Why does it have three wheels?" "Does it have brakes?" "And gears?" "Can you go fast on it?" and "Can I ring the bell?" among other questions. He sat on it but, fortunately, could not reach the pedals. If he had been able to reach the pedals we probably would have had to pick him up from the far end of the platform - rather a long distance away.
When we finally set off he is asking more questions, "Why does this one go here?", "Where is that one going?" "How do they know where each one goes?" and then, as we come out into the daylight of the railway yard and he can see all the tracks, "Why do the wheels stay on the rails?"
and more and more questions. His mother cannot answer them and does not pretend to know.
I do not know the answers to all of them either.
Then diagonally opposite in another seat an elderly man stops reading a document, folds it and puts it away. He looks across and says quietly,
"May I help? I used to work for the railways."
He shifts across into a seat next to the small boy and starts to answer all his questions all over again.
The mother talks to me. It is the first time they have been on a train. The ride is an adventure. They are going "all the way" which means to the Belair station in the hills. There will be two tunnels and an interesting cutting as well as the stops and starts at stations. "Why does the train stop if nobody comes or gets off?" "Can it go faster than a car?" "How fast can it go?"
When we reach my station, which is a changeover point for the single track working, they have to wait for the down train and I hear the elderly man explaining why this happens. Then he says, "When we get to Belair would you like me to ask the driver if you can have a look in the cabin?"
There was the silence of suppressed excitement as I left the train. I wonder how long it lasted.