Monsoon rains stranded nearly 100,000 Bangladeshi islanders in water up to their chests Wednesday, officials said.
Four feet of water was being held back by a berm built to keep out the sea on the island of Sandwip in the Bay of Bengal, they said.
Torrents of rainwater swept away a small railway bridge, disrupting train service between the island's northeastern Sylhet district and the port city of Chittagong, officials said.
Several villages in Sylhet were also inundated after a flood-protection embankment was breached, they said.
Heavy rains have pushed three rivers above danger levels in northeastern Bangladesh.
Thirteen inches of rain fell in the region in the 24 hours leading up to Wednesday morning.
The monsoon season lasts from June to September in Bangladesh.