The dream of wage gap discrimination takes another hit.

From Why Do Women Earn Less Than Men? Evidence from Bus and Train Operators by Valentin Bolotnyy and Natalia Emanuel.

Social Justice theorists worldwide desperately want there to be a gender income gap based on gender discrimination. The desire is frustrated because study after study, decade after decade, keep showing that it is personal priorities and choices which determine income levels, not gender discrimination.

This is the latest, with a moderate twist. Last year someone did a study of Uber drivers who are self-dispatched, i.e. each driver selects when, where, how long to work. There is no human agency to discriminate against a driver. Under those circumstances, the researchers found there was still an income gap of several percent based on personal driver choices.

This study is somewhat similar. From the Abstract:

Even in a unionized environment where work tasks are similar, hourly wages are identical, and tenure dictates promotions, female workers earn $0.89 on the male-worker dollar (weekly earnings). We use confidential administrative data on bus and train operators from the Massachusetts Bay Transportation Authority (MBTA) to show that the weekly earnings gap can be explained by the workplace choices that women and men make. Women value time away from work and flexibility more than men, taking more unpaid time off using the Family Medical Leave Act (FMLA) and working fewer overtime hours than men. When overtime hours are scheduled three months in advance, men and women work a similar number of hours; but when those hours are offered at the last minute, men work nearly twice as many. When selecting work schedules, women try to avoid weekend, holiday, and split shifts more than men. To avoid unfavorable work times, women prioritize their schedules over route safety and select routes with a higher probability of accidents. Women are less likely than men to game the scheduling system by trading off work hours at regular wages for overtime hours at premium wages. These results suggest that some policies that increase workplace flexibility, like shift swapping and expanded cover lists, can reduce the gender earnings gap and disproportionately increase the well-being of female workers.

It is something of a natural experiment. The dynamics of union-management interaction lead to highly proscriptive processes which substantially mediate management discretion.

It is all about personal choices.