North American freight railroads are expected to face increasing capacity constraints due to substantial, expected long-term growth in traffic. In order to meet this forecasted demand railroads must efficiently use existing capacity and effectively plan new capacity. Infrastructure to provide this capacity is capital-intensive so careful consideration of alternatives to infrastructure expansion must be considered. Consequently, railroads need to understand how operational practices and new technologies may affect rail line and network capacity. A comprehensive literature review of previous work on railroad capacity was conducted and the various metrics used to measure it described. An assessment of the major Class 1 railroads' capacity planning methods was conducted and their research needs identified. Operational factors influencing capacity are identified and described. Rail capacity is often measured using train delay as the metric so the categories and sources of delay were evaluated. Train type heterogeneity is a significant factor affecting railroad operating capacity. The relationship between delay, traffic volume and train type heterogeneity was investigated in a series of experiments using simulation analysis of trains operating on a single-track rail line. The specific types of conflicts and operational factors affecting delay were identified and quantified. Various operational and infrastructure methods to reduce train delay were analyzed and cost benefit analyses were conducted to determine their relative cost effectiveness. A qualitative analysis of the impact of positive train control (PTC), communications based train control (CBTC) and electronically controlled pneumatic (ECP) brakes was conducted. Each aspect of these technologies with the potential to affect capacity was identified and its effect evaluated under various implementation scenarios, including consideration of the conditions under which each technology has the potential to increase, reduce, or have no effect on capacity.