I have tried to do my homework regarding obtaining Kyat. Apart from crisp dollar bills. Is there anything else that can be used to obtain kyat at street rates.

I assume street rates are only obtainable in country. I notice kyat are available at some of the money changers in bkk.

I am looking at a train trip to Mawlamyine. Some of the recent YT videos look promising

Thai baht presently trading around 128-130 works ($ is 4200-4300). I used to buy US$ but these days just take baht which not only offer a comparable exchange rate but also do not have to be pristine. You can change in Bogyoke market (closed Monday) or at Eva Trading on 21st street but whatever your choice avoid the touts around Sule Pagoda as you will most certainly get ripped off. I am a frequent visitor and have never considered buying kyat in Thailand.

Can't help with train information but if you are looking for a quality hotel downtown Rangoon consider the Excelsior, single around $60.

Safe travels.