My flight arrives to Yangon at 10:00 and I want to go directly to Mandalay this night by train. Normally when I do something like this I leave my backpack at the train station or somewhere else, but in this case I cannot find any information about luggage lockers in the station or other places in Yangon. Are there any?
The consensus online seems to be that, at the time of writing, there are no luggage lockers around Yangon. Your best bet would be to ask a hotel to keep the luggage for you. They will most probably ask for a fee. Even better would be to leave your luggage in a hotel where you plan on staying once you come back to Yangon, if applicable. Other options include hire a car with driver for the day and leave the luggage in there, and leaving your luggage at the bus station ticket counter. Both options will cost you.