It's not entirely true that there were no bombings. Around Auschwitz-Birkenau camp there were several major oil refineries, which converted Silesian coal to fuel and were bombed several times. My grandfather, who still lives, was an antiaircraft gunner, then a radar engineer in the Wehrmacht for six years and was stationed there for several months in 1944 in a flak battery. "Their" refinery was bombed several times, and on at least one occasion the railway leading to the camp was destroyed. Their own barracks were adjacent to Auschwitz II camp, and he speaks about the camp frequently. Apparently before they arrived, soldiers had no idea about what was going on in the camp, but they understood it the first evening which understandable came as a fundamental shock. I believe him that they didn't know it beforehand, because he wouldn't have a reason to lie. After several months he couldn't stand it anymore and asked to be transfered somewhere else. (After an interlude in jail, they apparently found that radar engineers were too valuable to waste and sent him first to his native Ruhr area, which was under constant attack, then to Dresden where he served on an inner-city antiaircraft battery during the February bombings.)
It seems to have been surprisingly difficult to destroy railway installations from the air; Germany basically had a rudimentary, but functioning railway infrastructure until the end of the war despite stations being under constant attack.