Rail was the main means of transport used by the Germans for deportation to Auschwitz. In 1940–1941, trains stopped at a siding adjacent to Auschwitz I. They carried mostly Poles. A second siding, known as the Jewish ramp (German: Judenrampe), was located at the halfway point of the 3‑kilometer distance between Auschwitz I and Birkenau. Transports of more than half a million people, mostly Jews, arrived there from the spring of 1942 to mid‑May 1944. It was the place where SS doctors conducted the selection of new arrivals; strong, healthy people were sent on foot to the camp, while those regarded as unfit for labor were taken by truck to the gas chambers. In the spring of 1944, shortly before the arrival of about 400,000 Jews from Hungary (Sonderaktion “Ungarn”), the ramp located directly in the Birkenau camp took over its role.

(Mini dictionary of terms from the history of Auschwitz)