Increasing public transit ridership can minimize transportation-related greenhouse gas (GHG) emissions in areas with high automobile usage. However, there is limited literature comparing the life cycle impacts of transit alternatives, particularly low-cost options like bus rapid transit (BRT). New York City has identified a new transit corridor to be implemented in Staten Island, New York. The city will be constructing a bus rapid transit line or a light rail transit (LRT) line. This study uses life cycle assessment methodologies to quantify the expected GHG emissions associated with the construction and use of these transit alternatives. An assessment was performed to determine GHG emissions associated with the construction and operation of both alternatives. The use-phase was then analyzed according to the GHG emissions per passenger mile travelled, based on scenarios depicting ridership. The BRT alternative resulted in higher overall GHG emissions associated with the construction and operation of its components. Despite having higher projected ridership, the BRT system still had higher GHG emissions per passenger mile travelled. For both the BRT and LRT alternatives, the operational phase of both the BRT and LRT lines would cause the majority of lifetime GHG emissions. The results of this study can help compare the expected GHG impacts of bus rapid transit and light rail within the same transit corridor.