#9 Train Bangkok to Chiang Mai 1st Class Sleeper Ticket

Hi, I was hoping maybe someone here could help me: I’m trying to book a 1st Class Sleeper ticket on train #9 from Bangkok to Chiang Mai on February 19th.
However, I can only find the option of booking 2nd Class Sleeper tickets online. I usually book my local Thailand travel onsite when I’m in the country so I don’t have experience with the online booking systems. Am I not seeing the option for the 1st Class Sleeper ticket because it is sold out or am I looking on the wrong sites?
Thanks in advance.

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