I rode Lilly Belle September 10 - I didn't book, but got there at just the right time and was able to ride with an off-duty engineer who happened to be there and volunteered to take a group. I was there by myself, so it was probably easy to add me, but if you have more people it would be more difficult. It was really hot that day, and they specifically warned us before we boarded about the heat. It was a great opportunity, and our volunteer tour guide talked the whole way around telling us about all sorts of things about how the trains work, bits of history, the workday of the engineers, how he got his job, etc. He also made sure we all got a ticket afterwards, too.
He did say they don't have any more carpet for refurbishments, so when that wears out, Lilly Belle will be out for a complete overhaul, and will probably come back looking a bit different. In order to make it last as long as possible, they won't bring it out if there is any chance of rain, or anything that might cause damage or additional wear. If it's too hot, it will probably still be out, but not open for rides.