The current institutional process for project-level environmental review, the government-required Environmental Impact Statement (EIS), requires assessment of the proposed project, the no-build alternative, and alternatives to the proposed project. Despite growing academic research to compare the environmental impacts of air and high-speed rail (HSR) infrastructure, there are few instances of multimodal alternatives analysis in airport and HSR EIS documents. In this paper, examples of EISs for air and HSR capacity-enhancement projects are chronicled to identify key challenges to completing modal alternative analysis in the EIS: the spatial heterogeneity of the physical infrastructure for air and HSR, the framing of EIS purpose and need statements, and the complicated interpretations of environmental impact significance thresholds. The paper concludes by proposing strategies to incentivize modal alternative assessments and highlight methods that are needed to perform high-quality comparative analysis to inform decision makers, whether in the context of the EIS or in upstream planning processes.