BACKGROUND: Sudden cardiac arrest (SCA) strikes over 40,000 people in the public environment annually in the U.S., but despite evidence-based interventions such as prompt CPR and defibrillation, less than 25% of patients survive public SCA events. Effective use of automated external defibrillators (AEDs), especially by lay bystanders, represents an important strategy to improve survival rates. Previous investigations in Europe and Asia have demonstrated variable public awareness of AEDs; layperson knowledge of AEDs in the U.S. is poorly characterized. OBJECTIVE: To measure understanding of AEDs among the general public, at multiple sites within a busy urban transportation system. METHODS: Surveys were administered at two high-volume train stations in Philadelphia, Pennsylvania between April and June, 2013. RESULTS: A total of 514 surveys were completed. Two thirds (66%) of respondents were able to correctly identify an AED and its purpose, and just over half (58%) of respondents reported willingness to use an AED in an emergency situation. Less than 10% of respondents presented with a hypothetical SCA scenario spontaneously mentioned using an AED when asked what actions they would take. CONCLUSIONS: In this cross-sectional survey, public knowledge about AEDs and their use was high; however, a smaller number of respondents expressed thoughts of using the device in an emergency situation and demonstrated willingness to serve as a responder. Increased education and training efforts, as well as potential interventions such as 911 dispatcher-assisted AED use may help improve bystander response in SCA events.