The explosion occurred at 6:00 a.m., as the train passed over the Crimea Bridge, and was heard for miles. Russian officials suspect that it is a Ukrainian truck bomb, but deny it externally. Officials in Ukraine: "This is just the beginning", "A birthday present for Putin"
The Crimean Bridge is considered one of the symbols of the Kremlin's occupation of Crimea and is known to be hated by many Ukrainians.
The railway spans two parallel bridges and crosses the Crimean Strait that connects Krasnodar in Russia and Crimea - and was established by Russia after the invasion in 2014.
The 19 km long bridge was opened in 2018 by Russian President Vladimir Putin.
During the nine months of fighting in Ukraine, Russia assumed that Crimea, including the Crimean bridge, was beyond the capabilities of Ukrainian forces.