There is a large difference between bombing a military target and a civilian target. And that's true even if the "same" railroad can be either one or the other.
In military bombing, the idea is not to destroy the target per se, but to "put it out of action" during a critical time period, say right before a battle. For instance, the Allies bombed a lot of roads and railroads in France in connection with the Normandy campaign. A number of German units were "delayed," but they made it to Normandy eventually. The real objective of the railroad bombings is the Allied lives and time saved by the delay of German reinforcements. Without this consideration, the difficulty and cost of bombing a target would outweigh the value of the bombing itself; most roads, railroads, and buildings are easily repaired.
In the case of the railroads to Auschwitz, there were no military "savings" to offset the cost of the bombing. The railroads would have been repaired eventually, the shipping and killing would have gone on, and the main result is that people who were "shipped" to Auschwitz would either have been killed in the bombing, or forced to detrain and walk the remainder of the way, resulting in more and earlier deaths.