  The master equation approach is proposed to describe the evolution of
passengers in a subway system. With the transition rate constructed from simple
geographical consideration, the evolution equation for the distribution of
subway passengers is found to bear skew distributions including log-normal,
Weibull, and power-law distributions. This approach is then applied to the
Metropolitan Seoul Subway system: Analysis of the trip data of all passengers
in a day reveals that the data in most cases fit well to the log-normal
distributions. Implications of the results are also discussed.

Physics and Society;Statistical Mechanics;Data Analysis, Statistics and Probability