I found post from 2008 that said not, but wondering if things have changed at all...

If the cat's contained then surely not a problem.  

Not so sure about cats but I'm certain that blue pigs can travel without any problems.

https://www.bts.co.th/eng/faq/faq-howto.html

 

This says no.

But I would use the "contact us" option to ask whether they can be carried in a suitable box or backpack.

A troll post has been removed.

Officially not allowed.

 

If you have the cat in one of those carry cases that don't look like a pet carry case you stand a very good chance of just being able to board. But should you be stopped for a bag check, 50-50 they don't let you on.

 

It will also be mega stressful for the cat, the noise and vibration will utterly freak them out.  Remember cats are extremely sensitive to vibrations.

 

Cab/Grab or bus (outside rush hours so not crowded) a better option. There probably is a rule somewhere saying no pets on buses but they don't check bags and a well secured cat isn't going to be a problem. Cat won't like bus either,  but not as traumatizing as BTS.