Does anyone have experience traveling on the Bangkok-Chiang Mai night train?

According to the Thai Railway schedule, there appear to be two Special Express overnight trains operated daily, both with First Class sleepers and two additional Express trains with Second Class sleepers. Besides the differing accomodations, are there any differences between these four night trains?

Also, the Thai Railway online ticketing website doesn't seem to allow the booking of First Class sleepers - only Second Class and below. Does anyone know how I can prebook a First Class sleeper for two before I arrive in Thailand?

Thanks,
FlyerBeek