During the German occupation of Poland in World War II, Ząbecki worked as a dispatcher for the Deutsche Reichsbahn; he also became a secret soldier in the underground Armia Krajowa (AK), collecting classified data and reporting to the Polish resistance on the Holocaust transports that went to Treblinka extermination camp.

His incriminating evidence against them included original German waybills produced by the Reichsbahn, which proved that the "Güterwagen" boxcars crammed with prisoners on the way to Treblinka were returning empty.

There he secretly joined the resistance under the nom-de-guerre "Dawny" (the old-timer in Polish) and was asked by the Armia Krajowa (AK) to keep a watch on the German rail transports passing through the station.

[4][5] In his book of facts about the camp history titled Wspomnienia dawne i nowe (Old and New Memories), Ząbecki estimated that no fewer than 1,200,000 people were murdered at Treblinka.

[4] Even though these estimates have been revised by others in the following decades, his book is still one of the leading sources of information for professional historians about the mass deportations to Treblinka from the Jewish ghettos in German-occupied Poland, their frequency, and volume.

Notably, when Ząbecki went to Düsseldorf to testify against him, he was offered a good life in the West by the defense attorneys in case he chose to defect.

[9] During the war, Ząbecki secretly watched the railway line from Siedlce to Małkinia Górna junction, which was shown in the film together with his clandestine work for the underground, and his daily recording of data.