While public transit systems are perceived to produce lower GHG emission intensities per passenger miles traveled (PMT) and per vehicle miles traveled (VMT), there is a limited understanding of emissions per PMT/VMT across cities, or of how emissions may change across modes (light, metro, commuter, and bus) and time (e.g., with changing electricity mixes in the future). In order to better understand the GHG emissions intensity of public transit systems, a comparative emissions assessment was developed utilizing the National Transit Database (NTD) which reports energy use from 1997 to 2012 of rail and bus systems across the US. By determining the GHG emission intensities (per VMT or per PMT) for each mode of transit across multiple years, the modes of transit can be better compared between one another. This comparison can help inform future goals to reduce GHG emissions as well as target reductions from the mode of transit that has the highest emissions. The proposed analysis of the NTD and comparison of modal emission intensities will be used to develop future forecasting that can guide public transit systems towards a sustainable future.