The phrase "divided along racial lines" refers to a situation where people of different races or ethnicities are separated based on their differences. The term implies segregation and discrimination, highlighting the existence of deep societal divides that can cause injustice. In rephrasing this phrase, you could use terms like "segregated by race," "experiencing segregation due to racial differences," or "split by ethnicity." Other synonyms might include describing a "racial separation," "ethnic divide" or "cultural boundary" that separates people. Whichever phrase is chosen, the goal is to accurately convey the situation of people being separated or discriminated against on the basis of race or ethnicity.