How do you tell your manager that you don’t want to sit next to him on the train?

I get on really well with my manager. He’s not a dick. We’re getting the same train down to a corporate sales event soon. We booked separately and have reserved seats in different coaches but apparently there are always lots of seats available so we don’t need to worry about that. He was saying it is a great time to discuss things and “just have a good old chinwag”. Nothing like going over spreadsheets first thing in the early morning. I’d quite like to be on my own for the journey. Once we’re there then my time is fair game. How can I politely say that I’d rather have the time to myself while travelling?