Nothing is ever made better or worse by anything, except from a subjective point of view. Arguing that European imperialism was good because of railroads or bad because of slavery requires one to have set preferences in favour or disfavour of one or the other, it's not at all a question about whether Africa was made better or worse objectively speaking but whether it developed according to our tastes. Today most people like railroads but not slavery, in the past you had people who liked slavery but not railroads. Suffice to say; Europeans brought both railroads and slavery and neither made Africa better or worse, merely different. From there on it's up to you to decide how to feel about those differences based on your own convictions.