"This attack was very systematic, significant and on several levels", says operator Ukrzaliznytsia in a statement.
The attack, which is now in its second day, is said to have primarily affected the digital booking system.
Ukrzaliznytsia is responsible for most of Ukraine's railways, including freight and passenger train traffic that connects Ukraine with other European countries. They also transport Ukrainian soldiers to the front lines in the east.
It is unclear who or what is behind the cyberattack. Technicians and the country's security service are said to be working to resolve the situation.
Ukrzaliznytsia has been severely affected by the war, with many employees killed in Russian attacks on various parts of Ukraine. Drone and robotic attacks have also repeatedly put the country's rail traffic on hold.
