Tusk said the blast, which occurred on Sunday near the village of Mika, about 100 km southeast of Warsaw, destroyed a section of track and appeared to be "an act of sabotage aimed at derailing a train."
A second incident was reported later on Sunday, when a train carrying 475 passengers made an emergency stop after its windows were shattered, likely by a damaged overhead power line, state news agency PAP reported.
"No one was injured," the Lublin police department said, adding that officers, prosecutors and security services were working at the scene.
Investigators from the Central Bureau of Police Investigation (CBŚP) and the Internal Security Agency were also deployed.
Route 'crucially important for delivering aid to Ukraine'
Tusk called the track explosion "an unprecedented act of sabotage targeting the security of the Polish state" and its citizens.
"This route is also crucially important for delivering aid to Ukraine," he said, vowing efforts to "catch the perpetrators, whoever they are."
The government convened an emergency meeting of ministers and security chiefs on Monday morning.
Interior Minister Marcin Kierwiński said there was "no doubt" that the blast near the village of Mika was deliberate, adding that another section of the strategic rail line was also being examined for damage.
Tusk, Kierwiński and Tomasz Siemoniak, the government minister overseeing the country's security services, visited the site of the explosion early on Monday.
Security officials said the Internal Security Agency (ABW) was leading the inquiry in cooperation with police, prosecutors and railway services.
Defence Minister Władysław Kosiniak-Kamysz said the military was assisting in the investigation and asked to examine some 120 kilometres of track toward the Ukrainian border for any irregularities.
Territorial defence forces from the eastern Lublin region were taking part in the operation, he said.
President Karol Nawrocki was being kept informed by his National Security Bureau (BBN), which said the incidents showed signs of deliberate sabotage and required "clear and responsible communication" from all state institutions.
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Source: IAR, PAP, TVP Info