CRRC is about to open its rail car factory in East Springfield, MGM Springfield is seven months from opening its resort casino in the South End and now homegrown MassMutual will add jobs at its headquarters on State Street. “That is a lot of momentum, and it gets noticed,” said Richard K. Sullivan Jr., president and CEO of the Economic Development Council of Western Massachusetts. MassMutual plans to grow its workforce at its corporate headquarters on State Street by approximately 50 percent. The insurer will add 1,500 jobs there over the next four years, bringing the total number of employees in Springfield to approximately 4,500 by 2021. MassMutual also plans to spend $50 million on its Springfield headquarters, which is only about 60 percent occupied, over the next five years. Sullivan said his job now is to use that momentum to bring in more business and more jobs.