The introduction of new mobility options has changed travel behaviors in U.S. cities, resulting in major shocks to transit agencies’ core operating principles and bottom lines. These changes have resulted in some benefits, but have also put a strain on transit ridership, infrastructure, and ongoing efforts to curb emissions and energy use. The Chicago Transit Authority (CTA) is interested in collaborating with the Argonne National Laboratory to leverage their transportation modeling and simulation capabilities to evaluate regional travel behavior against several possible scenarios. Using Argonne’s agent-based transportation model and high performance computing (HPC) resources, this modeling effort will help CTA better understand the complex interplay between the many variables affecting CTA’s ridership and daily operations, and will support future decisions related to pricing strategies, investment prioritization, fleet electrification phasing, the equitable distribution of transit service, and other organizational and operations strategies. The outcomes of these decisions will shape the future of transit service in the Chicago region and will have profound impacts on the region’s energy resiliency.