Take a sweater or a blanket to cover yourself, air conditioning is sometimes too strong in the train. If you can brush your teeth, you can not take a shower on the train. In general, travelers prefer the lower sllepers. And apart from the bustling restaurant car, the train is quiet and you can sleep pretty well (if you can forget the noise of the railway (or use earplugs).
But you can take the train at any other stop. For example an Ayutthaya - Chiang Mai trip by night train will enable you to take the time to discover the ancient capital of the Kingdom before boarding to the northern capital.