To address the limitations of existing studies regarding analytical scales and the exploration of social equity issues, an accounting–analysis–application framework is constructed to uncover the travel carbon emission patterns of urban public transportation. Based on Beijing’s smart card data, a bottom-up accounting model for travel carbon emissions at the street scale is developed. Through a three-dimensional equity system (vertical, horizontal, and spatial), the relationships between emission characteristics and socioeconomic factors are analyzed, and multi-scenario shared bicycle substitution schemes are designed to evaluate the emission reduction potential. The results show that travel carbon emissions exhibit distinct spatiotemporal patterns characterized by tidal polarization on weekdays, multi-core dispersion on holidays, and a mixed transition on rest days. Equity analysis reveals a significant phenomenon of environmental cost shifting: during peak hours, low-housing-price areas contribute half of the total travel carbon emissions, with per capita emissions 72% higher than those in high-housing-price areas, resulting in a spatial lock-in effect of low-income–high-emission and high-income–low-burden. Furthermore, shared bicycles demonstrate the optimal emission reduction effect for medium-distance trips of 2–4 km; scenario synergy can achieve an emission reduction of 4.24% and effectively promote transport environmental equity by precisely alleviating the carbon burden in peripheral communities. This study provides a systematic analytical framework and fine-grained policy references for the equitable, low-carbon transition of urban transportation.