The MBTA has an employee shortage, unable to replace as many as 500 employees who have departed since 2015, although this year the trend appears to be reversing itself.

The T’s Fiscal Management and Control Board received a presentation Monday saying the agency has hired 2,520 people since 2015, but over that same time period 3,052 employees have left, Commonwealth Magazine reported. Retirements take up a large share of the departures, at 44 percent, but 56 percent of the departures were due to employees finding other jobs or being terminated.

The agency’s new head of human resources pledged to get to the bottom of the situation, the magazine reports, and raised the possibility that departing employees may be able to get better pay elsewhere, that they may feel unable to advance in their careers or that they may be not adequately performing their jobs. Many MBTA employees also took advantage of a voluntary retirement incentive program in 2017.