I would not count on the train to arrive on time if I had a flight to catch and make a ground connection to BKK.
Although, If you do enjoy train rides as I do, it is a comfortable enough way to get to Chiang Mai. Unless you are tall as, you may find the bed a bit short for you. Leave Bangkok in the evening and arrive in the morning in CM. The rolling hotel works for me. There is some food available onboard or you can bring your own.
The new cars look nice and modern. Will sure miss those commodes where you can see the rail bed zip by. Old world charm!