After the Polish Army was mobilised in 1939, the train was assigned to the Poznań Army. In the first days of the war, the "Danuta" supported various Polish infantry units. On 4 September, the train was bombed by the "Luftwaffe", but received only minor damage. Next the train participated in the Battle of the Bzura. On 16 September, the train helped to halt the advance of the German 24 Division and tried to withdraw towards the Polish 16 Infantry Division, but was ambushed by the German anti-tank artillery. The damage received in the ambush and the fact that the train's ammunition supplies were almost depleted forced the commander of the train, Captain Korobowicz, to order the train to be blown up, together with the assaulting German infantry, to prevent German capture. Out of the tanks used to scout on and off the rails, five tankettes TKS were evacuated successfully and two FT-17 tanks blown up.