Commuter rail boardings are more strongly influenced by CBD employment density because these systems usually have a single downtown terminal. Higher-density CBDs assure that more jobs are within walking distance of the commuter rail station. Throughout the study these effects are measured holding constant transit system characteristics such as parking availability, station distance to the CBD and station area income levels. Moreover, the higher speeds and longer distances on commuter rail tend to increase ridership to the CBD from precisely those places outside the city where residential densities tend to be low. In this study, cost-efficiency is measured by annual operating costs plus depreciation per vehicle mile.
Effectiveness is measured by daily passenger miles per line mile.
For light rail, these measures indicate a strong positive relationship with CBD employment size and residential density. This suggests that medium to large cities with higher density corridors work best for light rail. The length of the rail line is important for both light rail and commuter rail. Longer light rail lines are both slightly more cost-efficient and effective, but ridership diminishes beyond 10 miles. Commuter rail lines are much more cost efficient when they are longer, but their effectiveness declines beyond 50 miles.