Myanmar has one of the most extensive rail networks in Southeast Asia, though the exact length in km is unverified. The country is currently in a civil war, and the railway is used by the military junta for transportation.

Railway operator: Myanma Railway.
There is no English website and the site tends to go offline frequently. There is an archived English page at https://www.railways.gov.mm/en_US/.

Wikipedia: Rail transport in Myanmar.