Are any of his book suitable as casual on-the-tram reading? Could you recommend one of his books? Thanks.

I wouldn't say any of his books are exactly casual reading. The Large the Small and the Human Mind is probably the most casual in that it merely requires thinking and even then some of that is done for you the responses from Abner Shimony, Nancy Cartwright, and Stephen Hawking (contributing authors). The others require active engagement to work through. Especially Road to Reality - there is nothing casual about that one.

I started reading Road to Reality very casually– bought the book one day, next day I was in a Starbucks enjoying it and my tea. 10 minutes later one of the employees takes a break and the only table open is next to mine near the back. After a few minutes of sighing, stretching, and looking at the door to see if any new customers were coming in he turned to ask what I was reading. The next 30 minutes was the first and only time I have ever had a discussion about particle physics with a barista.

Depends how strong you are - I have a copy of "The Road to Reality" on my desk at work, but not sure I'd want to carry it about!

All books are suitable for casually reading on trams, apart from really old and delicate or oversized ones.

I loved 'The Emperors's New Mind' and 'Shadows of the Mind', and did read some of those two while on train journeys, if that helps at all.

The Road to Reality, while certainly not a technical work in the traditional sense, is not light reading.