The heads of the allied powers realized that the only way to stop the Nazi murder machine was to kill it in its entirety and consider it as an organic entity. This meant that it had to be starved to death. Bombing the camps was controversial because the primary casualties would have been the inmates. The Allied commanders decided that the most humane strategy would be to bomb the German source of oil, the Rumanian oil fields at Polesti, Rumania, however, the fields were too heavily defended and more damage could be done to the German infrastructure by fire bombing German cities. As for bombing the rail lines, well, it just does not work. The Wehrmacht had hundreds of units known as eisenbahn bautruppen, or railroad repair battalions. Here is an example which should suffice: the last major german offensive on the eastern front was the Kursk offensive in July 1943. The Soviets were well prepared for it as the British, who had been decoding German enigma machines for years by this date told the Soviets the exact starting date of the offensive. This information was passed to the Soviet partisans who the night before the offensive cut the German rail lines in over 5000 places. Within a week the lines were repaired. Also, the Nazis knew only that the Auswitz/Birkenau complex was huge, but they bought the fiction that it's primary mission was war industry output and not murder. Even the allied commanders could not conceive the purpose of the Nazi murder factory system -- to eradicate and only to eradicate human life.