This dissertation investigates the political economy of low-carbon infrastructure in cities in low- and middle-income countries. It focuses on the urban transportation sector, which is projected to emit alarming amounts of greenhouse gas (GHG) emissions by mid-century. Since most transportation emissions come from private automobiles, the more that urban transportation systems mitigate private vehicle dominance today, the more GHG emissions can be avoided tomorrow. Otherwise, the danger of carbon lock-in will loom large. Build-out of car-centered transportation infrastructure could trigger its continued expansion and set many cities on GHG-intensive pathways of urban development. The theoretical question at the heart of this dissertation therefore is: When, how, and under what conditions do cities deploy and maintain low-carbon infrastructure systems? This research argues that Bus Rapid Transit (BRT) systems, a bus-based mass transit innovation, offer a unique empirical setting to investigate this question. For one, BRTs demonstrate an empirical track record of effective GHG mitigation. For another, they have proliferated in cities across the Global South since the mid-2000s and thus afford a data-rich starting point for analysis. The empirical question at the heart of this dissertation therefore is: When, how, and under what conditions do cities deploy and maintain BRT systems? Current theoretical frameworks cannot yet explain the distribution of BRT systems across cities in low- and middle-income countries. This dissertation integrates insights from three broad academic literatures to examine the role of political institutions, political interests, and public investment. These three literatures, broadly defined, include works on Cities and Climate Change, the Political Economy of Public Outlays and Infrastructure, and BRT Studies. In When Do Cities Deploy Low-Carbon Infrastructure? Evidence from the Global South (Chapter 2), this dissertation assesses which macro-political institutions are associated with the adoption of BRT systems. To this end it leverages a novel dataset of 92 BRT adoptions between 1970 and 2017, and examines whether political decentralization and democracy, alongside international non-governmental organizations, are associated with greater chances of BRT deployment. The results from three sets of survival regressions suggest they are: politically decentralized countries are significantly more likely to adopt BRTs when they are governed democratically. The analysis does not find the same association in autocratic government contexts. Moreover, countries which host international non-governmental organizations advocating for BRT are indeed more likely to adopt domestic demonstration projects. This highlights that international civil society can serve as a driver of low-carbon urban innovation. In Governing Rapid Transit: Power Politics and Adaptability (Chapter 3), this dissertation then further contextualizes the findings from the survival regressions by analyzing how political interests, together with political institutions, shape the deployment and durability of BRT systems. It develops two analytical frameworks for how to rethink transit planning and policy. First, the deployment of BRT systems can be understood as the result of power politics. BRT proposals tend to pitch broad yet shallow support against narrow yet fierce opposition. This renders them inherently contentious. Proposals move forward, however, when their implementation generates political benefits for powerful individuals. Second, the durability of BRT systems can be understood in terms of adaptability to adverse policy feedback. BRTs often suffer from cycles of adverse policy feedback: operational problems lead to backlash (e.g., from users, car owners, elites), which can exacerbate operational problems and lead to more backlash. Over time, such cycles result in financial instability, loss of social legitimacy, or even closure. BRTs do not necessarily endure because their operations perform well. Rather, contrary to the dominant view found in the literature, this research suggests that they survive when they successfully adapt to and mitigate adverse policy feedback. The chapter offers a novel analytical vantage point for understanding transit planning and policy, complementary to more conventional policy frameworks. It develops the two frameworks with evidence from 32 expert interviews conducted between April and June 2019. In Institutional Foundations of Rapid Transit Investment: Evidence from Mexico, Peru, and Colombia (Chapter 4), this dissertation investigates how political institutions and interests affect public investment for mass transit development. It argues that there are four meso-political institutional enabling factors: an institutional actor who legitimizes and authorizes transfer of state capacity and resources to sub-national projects, a framework which facilitates the alignment of national and sub-national policy interests around shared policy goals, a politically savvy funding mechanism which disciplines public authority, and a mechanism of equitable fiscal redistribution. Taken together, these factors help accelerate rapid transit development, especially when there is a policy gap between sub-national regulatory responsibility over urban transport affairs and sub-national capacity to implement transport reform. This argument is examined with a comparative case study of public investment for BRT in two middle-income democracies in Latin America: Mexico and Peru. Mexico adopted 11 BRT systems between 2003 and 2017, whereas Peru deployed only a single BRT corridor within the same timeframe. The analysis shows how the four factors facilitated the creation of a national investment program in Mexico, and how the absence of the four factors in Peru has made mass transit development comparatively more difficult. To strengthen the external validity of the argument, the analysis also triangulates supplementary evidence from Colombia. Its methods combine process tracing, historical analysis, and 30 semi-structured interviews conducted between June and December 2021. The chapter offers a novel institutional explanation for why Mexico and Colombia, unlike Peru, have been able to scale up public investment for mass transit development. This dissertation concludes with several avenues for future research and policy implications. Overarchingly, it contributes to scholarly debates on how to accelerate urban energy transitions, the politics of mass transit policy and planning, and the political economy of low-carbon infrastructure and investment.