From BR Bullpen
The taxi squad was introduced to Major League Baseball in 2012 as part of the 2011 collective bargaining agreement. It allowed teams to add a 26th player to their major league roster when the team was involved in a doubleheader, as a way to give the clubs extra depth. The New York Mets added Jeremy Hefner for an April doubleheader using the new role. The players are only allowed to be on the MLB roster for 24 hours, after which they are returned to the minors.