I was sitting on a train with my pad and a pen, trying to write a poem. I had no title, but I had written down the first line

...I was sitting on a train with my pad...

A man sat opposite me.
After a minute or so of scanning his paper and throwing cursory looks in my direction
he enquiried "What are you writing?"

"I'm trying to write a poem about a man trying to write a poem on a train
who gets asked by a stranger 'what are you writing'.

"Can I be in it?", asked the stranger opposite.

"You already are", I replied.

The train pulled out of the station.