Final answer:
The Supreme Court upheld Louisiana's Separate Car Act by focusing on upholding racial segregation laws, under the principle of "separate but equal" from the Plessy v. Ferguson decision.
Step-by-step explanation:
In Plessy v. Ferguson, the Supreme Court was tasked with evaluating the constitutionality of Louisiana's Separate Car Act, which enforced racial segregation in railway carriages. They determined that segregation did not violate the Fourteenth Amendment's equal protection clause as long as the separate facilities provided to each race were equal, a principle that came to be known as "separate but equal." Thus, the Supreme Court's rationale in upholding the Separate Car Act primarily focused on upholding racial segregation laws, under the guise of providing equal accommodations despite the races being separate.