Travel to Chiang Mai, whilst every now and then I like the train it is very slow and as a norm most of the journey is at night, so you don't really get to see some of Thailand's wonderful land and city scape. I read the buffet car / food service has been reopened but how good it is post covid I don't know. Booze is still prohibited so it is not for sale.
Yes, we currently have quite a bit of pollution, not as bad as in previous years so I am not finding it prohibitive. Apart from that it is a wonderful time of year, still cool in the morning but not for long, mid 30's in the afternoon. quite a lot of life now, things to see and do.
If it was me, I would fly.