What are the implications of mass transit improvements for residential income segregation within cities? I observe large income differences in households‘ usage of and residential proximity to `fast‘ versus `slow‘ transit (e.g. subways versus buses on shared lanes). Consistent with these observations, I propose a theoretical framework to characterize the relationship between income segregation and the spatial distribution of transit speeds and travel mode choices within cities. I find that transit improvements that would maximize transit ridership tend to reduce income segregation when improving `slow’ transit but increase income segregation when improving `fast’ transit.