Churchill, Stalin and Roosevelt knew from the beginning what was being done to the Jews. They refused to act. They even refused to include what they knew of the genocide in propaganda against the Axis. They at the least could have announced that those that did the killing would be hunted down and punished. Nor would they bomb the rail lines that brought people to these execution camps.
Without getting into a discussion of the desirability of bombing the rail lines or camps, that tactic was first brought up by the Polish government in exile. Of course it was their citizens, Jews and Christians, who were being killed. As to propaganda, yes, it should have been used.
What would that have accomplished? The Germans were conducting mass killings of Jews before they built their first death camp in Poland. At Chelmno, they killed over 300,000 Jews, between 1941-45, by using gas vans - the rail lines didn't even reach Chelmno.
The Allies were committed to total war with Nazi Germany, to end the killing of innocents as quickly as possible. The Nazis, on the other hand, were committed to wiping out the Jews, and diverted essential resources from their war effort to that end, even when the war was obviously lost. Bombing rail lines to concentration camps wouldn't have saved any Jews, or stopped the Nazis. The Germans would have unloaded the stopped trains, machine gunned the prisoners and then gone back to kill more.