As trains are unreliable, arrival and departure times can vary. Yangon station has a digital departures board (in English), but few other stations offer this facility.
You will need your passport to buy a train ticket.
Many train journeys have multiple stop-off points, often allowing you to get out of the train, stretch your legs, and grab some refreshments.
Trains can get cold at night, so make sure to take warm clothes.
Buying train tickets can take a long time (sometimes as long as half an hour), as station staff go through the antiquated and bureaucratic form-filling that is required for foreigners, and friendly station masters take an opportunity to practice their English.
Even in upper class, seats can sometimes be a little smelly and greasy. Also, toilets are basic, and can be dirty.
Monks have free upper class travel; they are often very chatty and offer interesting conversation!
Trains, and transport in general, tend to be heavily booked during then Thingyan festival in April.