The Federal Reserve Bank of St. Louis seasonally adjusts this series by using the 'x12' package from R with default parameter settings. The package uses the U.S. Bureau of the Census X-12-ARIMA Seasonal Adjustment Program. More information on the 'x12' package can be found at http://cran.r-project.org/web/packages/x12/x12.pdf. More information on X-12-ARIMA can be found at http://www.census.gov/srd/www/x12a/.
Source: US. Bureau of Labor Statistics
Federal Reserve Bank of St. Louis and US. Bureau of Labor Statistics, All Employees: Transit and Ground Passenger Transportation in Massachusetts [SMU25000004348500001SA], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/SMU25000004348500001SA/, November 30, 2015.