If you look closely at the sign you'll notice that it's been rewritten. I wonder what the previous sign said. "Wait for Peter"?

Seriously though, a Shunter is the person responsible for switching the tracks onto which approaching trains will be guided. The process is called "shunting", hence the title Shunter.

I'm sure you can imagine that it really is a good idea to wait for the Shunter to finish shunting. :)