First of all, it was a Louisiana law that resulted in the case Plessy v. Ferguson, not an Alabama law.
Second, my reading of primary sources -- contemporary newspapers, biographies, and autobiographies -- tells me that black men and women traveled frequently by train. Well into the 20th century, trains were the only practical way to travel long distances.
Third, on north-south routes, segregation seems to have ended in Washington, D.C., if you were going north. (Conversely, it began in Washington if you were traveling south.) Traveling east, blacks were required to move to segregated cars in Cincinnati before the train crossed into Kentucky.
I suspect whether black passengers moved to "white" cars depended on the individual. There were still bigoted whites who might make a journey unpleasant, even in states were there was no segregation.
Fourth, while it is true that many Northern states had racially based laws, only in the South was segregation essential to the framework of white supremacy by circumscribing what blacks could and could not do and delineating in tiresome detail how the races could and could not interact. In some states, for example, blacks could not check out library books set aside for whites.
(This Wikipedia link gives examples of Jim Crow laws by U.S. state over a period of years: https://en.wikipedia.org/wiki/List_of_Jim_Crow_law_examples_by_state#California.)
Fifth, apologists for segregation liked to claim "separate but equal" facilities were provided black people. This was simply not true. Blacks schools in the South were funded at 1/10th the rate of white schools. Black teachers were paid half (or less) of what white teachers were paid. And, on trains blacks were relegated to substandard cars that lacked luggage racks. Bathrooms were inferior to those in white cars. In some cases, the cars set aside for black people were also the smoking cars. Worse, black women feared to travel alone because of the possibility they might be assaulted by white men in the "colored" cars.