Interior Minister Marcin Kierwiński told reporters that investigators had confirmed the first of two weekend incidents on the line as an act of sabotage.
"Beyond any doubt, an explosive device was detonated, damaging the tracks," Kierwiński said.
He added that officials had gathered "very substantial evidence," including footage from nearby cameras and physical materials from the scene, which he said would allow authorities to quickly identify the perpetrators.
The announcement came during a joint press conference with Security Services Minister Tomasz Siemoniak, Justice Minister and Prosecutor-General Waldemar Żurek and Infrastructure Minister Dariusz Klimczak.
Authorities say two sabotage attempts occurred on the line.
In the first, an explosion near the village of Mika, about 100 km southeast of Warsaw, destroyed a section of track in what Prime Minister Donald Tusk said was likely intended to derail a train.
In the second, a train carrying 475 passengers was forced to brake suddenly on Sunday after encountering track damage. No injuries were reported.
Kierwiński said two additional incidents were confirmed late on Sunday: damage to 60 metres of overhead power lines on the same route, and a metal clamp found fastened to the tracks "a few hundred metres" away.
The clamp had been cut through by passing trains, he said.
The incidents are being investigated by a joint team including prosecutors, officers from the Central Bureau of Police Investigation (CBŚP), and experts from the Internal Security Agency (ABW).
Siemoniak said the likelihood that the sabotage was carried out on behalf of foreign intelligence services was "very high," but added that investigators were examining multiple hypotheses.
He noted that the line is one of several used to move military support to Ukraine.
"We will ensure that the perpetrators and those who ordered this attack are brought before a Polish court," he said.
Żurek said a criminal investigation has been opened under two articles of the penal code— one concerning sabotage and another covering "attempted catastrophe in land transport."
No arrests have yet been made. If proven, the crimes carry penalties ranging from 10 years to life imprisonment.
Klimczak credited train drivers and railway staff with preventing a disaster.
He said repairs were underway near the cities of Puławy and Dęblin and that full two-track service could be restored overnight once investigators finish their work.
Railway operator PKP PLK and the Railway Security Guard have increased patrols and inspections across the network, officials said.
Poland operates roughly 19,000 kilometres of rail lines, each with established monitoring and security systems, Klimczak said.
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Source: IAR, PAP, TVP Info