According to Justice Harlan, what did the railroad claim was its motivation for the decision to separate passengers?

Justice Harlan wrote the dissenting opinion in the 1896 Supreme Court case Plessy v. Ferguson. This case stated that it was constitutional for states to have "separate but equal" public facilities, including schools, until it was overturned (related to schools) in the 1954 Supreme Court case Brown v. Borad of Education. 
In Plessy v. Ferguson, the court ruled that Louisiana could have separate railroad cars for black people and white people. In Harlan's dissent, he dismissed the idea that the railroad did not discriminate against either race but used the same rules to apply to both races, as they had claimed. Harlan wrote:

"The thing to accomplish was, under the guise of giving equal accommodations for whites and blacks, to compel the latter to keep to themselves while travelling in railroad passenger coaches. No one would be so wanting in candor as to assert the contrary."

In other words, Harlan said that under the pretense of giving separate accommodations to whites and blacks, the railroad's real motivation was to keep black people in their own coaches. The railroad intended to discriminate against blacks. Harlan wrote, "But in the view of the Constitution, in the eye of the law, there is in this country no superior, dominant, ruling class of citizens." That is, he said that under the Constitution, no race is superior and that all citizens have equal rights under the law.