Can I travel together with my girlfriend / boyfriend in the same cabin?

Yes, you can. Mixed gender is no problem on trains in Thailand.
If you book a single seat in a VIP compartment with double occupancy, you will be placed together with a person of the same gender.
And 'ladies only' carriages are for ladies only. Just in case you have missed this part.

Tags: Thailand, Thai Railways

Thai Railways

OK

I don't have a booking number. I have not purchased a ticket yet.