This study offers a combination of considerations and evaluation tools pertaining to relevant means of capacity improvements (technology, operations, route, and vehicle upgrades), both conventional and emerging. Guidance regarding the economics is offered to help balance the mix to minimize cost of achieving the level of capacity improvement required. The report describes principles and concepts related to capacity for heavy rail transit operations. Topics include track and station configuration, rolling stock, train operations, and signal and train control issues. Transportation Technology Center, Inc. (TTCI) identifies promising potential improvements and additions to infrastructure to increase capacity (emphasizing cost-effective technology solutions). Discussion is provided on investment planning to increase transit system capacity by making the various improvements noted. The study also discusses the benefits, effectiveness, and life cycle costs of the various solutions. A sequence for implementation of the various recommended changes is suggested. To illustrate these principles, TTCI evaluated various aspects of the present capacity limitations vs. ridership for two large rail transit systems in the United States to determine to capacity constraints and to identify areas where improved capacity might be needed. One section presents a limited case study of the Washington Metropolitan Area Transit Authority (WMATA) system. A second case study presents an overview of the Bay Area Rapid Transit (BART) system, along with a more in-depth analysis of BART operations and suggestions for capacity improvements. In each case study, analysis of delays shows areas where improvements could be made that would increase system reliability. Reduction in variability and unplanned events can provide not only increased capacity but a better passenger experience. Increased reliability and reduced delays and variability are keys to getting the most capacity out of existing systems. Analysis of train operations and model simulations for congested areas on one system point to the root causes of congestion. Changes and upgrades to train operations and train control systems are then simulated to determine effectiveness of measures to improve system capacity.