North American railroads are facing increasing demand for safe, efficient, and reliable freight and passenger transportation. The high cost of constructing additional track infrastructure to increase capacity and improve reliability provides railroads with a strong financial motivation to increase the productivity of their existing mainlines by reducing the headway between trains. The objective of this research is to assess potential for advanced Positive Train Control (PTC) systems with virtual and moving blocks to improve the capacity and performance of Class 1 railroad mainline corridors. Rail Traffic Controller software is used to simulate and compare the delay performance and capacity of train operations on a representative rail corridor under fixed wayside block signals and moving blocks. The experiment also investigates possible interactions between the capacity benefits of moving blocks and traffic volume, traffic composition, and amount of second main track. Moving blocks can increase the capacity of single-track corridors by several trains per day, serving as an effective substitute to construction of additional second main track infrastructure in the short term. Moving blocks are shown to have the greatest capacity benefit when the corridor has more second main track and traffic volumes are high. Compared with three-aspect signal systems, much of the benefits of moving blocks can be obtained from adding signals and implementing a four-aspect signal system. Knowledge of train delay performance and line capacity under moving blocks will aid railway practitioners in determining if the benefits of these systems justify the required incremental investment over current PTC overlay implementations.