As far as I am aware, there was only one battle where it happened. That battle was Stalingrad. The tank factory in question was the Stalingrad Tractor Plant (or Dzerzhinskiy Tractor Factory) now the modern day Volgograd Tractor Plant. There is in fact even a monument to the defenders of the plant. (a picture can be found here ) The Story behind the factory and the battle is basically is that when the German Army began to push into the city, the citizens of the city were not evacuated. The factory therefore kept producing tanks despite the city being under German assault. Eventually the German Army pushed further into Stalingrad reaching the plant. When the Germans attempted to capture the factory, the Soviet factory workers began to send tanks fresh off the assembly line into battle. Russia Today in 2010 published an interview with one of the workers in the plant. (Yes, I’m aware not the best source when it comes to bias, but I’ll take interviews with survivors from Stalingrad where I can get them.) In this interview worker Aleksandr Klemenko discusses his personal experience during the fight, he states, “Using just their own wheels, we sent them straight from the building into battle – without any adjustments, scopes or modification – to the front line, which was just outside the gate. Then, as the line pushed back we started using the railway again to get the tanks out,” (source here )