Economic models for rail systems

The economic justification of prospective railroad development is an example of a complex systems analysis problem. Many important aspects of the system economics cannot be assessed by examination of the vehicle alone; such factors as average train length and daily utilization are typical examples of extravehicular factors which must be accounted for. Consideration has, therefore, been given to the effect of vehicle parameters, such as capacity and speed, on the utility of a vehicle in a specified transportation system. To fully describe these effects it has been necessary to use computer simulation techniques to approximate the structure and operation of hypothetical rail systems. The methodology employs a computer logic model specially developed for this analysis. The model is general in nature and can be used to simulate any ground transportation system. The specific factors studied in a given situation may include: continuous variations in passenger demand; passenger trip profiles; train schedules; vehicle size, speed, and turnaround time; and right-of-way speed and headway restrictions. The model provides statistics on car utilization, load factor, required train sizes, and passenger waiting time. The model uses the IBM General Purpose System Simulator language (GPSS III). Although the model is still being developed, it has been used in preliminary analyses of new railroad equipment. For the cases considered, involving systems with three or four terminals, nine stations, and up to 75 trains per day, roughly two minutes of computing time was required to simulate a 24-hour day.