The incident occurred at the station in Puławy, in the eastern Lubelskie region, state broadcaster TVP Info reported.
Officers said they found the man between train cars at the time of his arrest.
Police said the suspect was carrying a bag containing mobile phones, electronic equipment, SIM cards and a power bank. He also had documents written in Russian, according to investigators.
They notified authorities including the National Public Prosecutor’s Office, the Military Counterintelligence Service (SKW), the Internal Security Agency (ABW) and the Central Bureau of Polish Investigation (CBŚP).
Security services were continuing operations at the scene.
Officials did not immediately say whether the man faces charges.
The announcement comes amid heightened security concerns driven by Russia's war in neighbouring Ukraine, with Poland intensifying its counterintelligence efforts.
The officers involved were participating in Operation Tor (Rail), a joint effort that includes police, soldiers from Poland’s Territorial Defence Force and the Railway Protection Guard.
Operation Tor was launched in response to acts of sabotage on rail lines, particularly along the Warsaw-Lublin route, with the aim of preventing further incidents.
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Source: TVP Info