People heading north from Bangkok tend to take an overnight train to Chiang Mai and don’t bother to stop to see anything along the way. If you have the time, I would recommend that you do a few stops on the way up. One good place is Lopburi which is famous for monkeys. I had heard a rumour that monkeys here sometimes hop on passing trains then return in the evening. I did find some monkeys watching the trains go by, but I never saw one actually get on the train.