The past 20 years have witnessed a remarkable period of growth for commuter rail. The potential to share existing infrastructure has made this mode of transportation an attractive solution to challenges posed by population growth and rising fuel costs. The increasing value placed on urban planning strategies that promote density has also boosted the attractiveness of this mode. This popularity has increased demand for high levels of service performance, defined by trip times that are competitive with automobile travel, and high rates of cost recovery relative to other transit modes. The development and the implementation of strategies that reduce travel time and improve asset use have become timely and appropriate considerations for commuter rail planners. This paper introduces the directness factor, a measure that can facilitate the evaluation of transit service alternatives through the synthesis of timetable characteristics that otherwise may require an extended explanation. Organizations may find the measure helpful if they are considering options for accelerated service, defined as service plans for which not all trains serve all stations. This measure can help identify the connections between timetable alternatives and system performance. For sketch-level planning exercises, for which limited information and institutional resources are available, the directness factor can be used to focus questions and to structure investigations that are more intensive than they otherwise would be. Within the current range of available planning and evaluation tools, the measure is intended to fill a gap between extended descriptions of service at stops on a specific line and modeled service demand and operating characteristics. This paper considered three specific implementations of accelerated service on American commuter railroads and used the directness factor to analyze the impact of those improvements.