Final answer:
The key infrastructure that enabled the SS to move millions of Jews across Europe during the Holocaust were the railroads, which were extensively used and sometimes expanded to serve concentration camps.
Step-by-step explanation:
The existing infrastructure that played a key role in helping the SS move millions of Jews throughout Europe was railroads. When the Nazis invaded Poland in 1939, they found the largest Jewish population in the world and the Holocaust began on an industrial scale. With the establishment of ghettos and concentration camps, the Nazis leveraged the extensive railroad system to transport Jewish people as well as other prisoners to these facilities. The systematic round-up of Jews and their deportation to concentration camps was facilitated by the existing railway networks, which had been expanded to meet the demands of the SS and the Nazi regime. In fact, some rail lines were constructed specifically to serve concentration camps, ensuring the efficient movement of prisoners to these locations of mass murder.